feat(web): Kalender som menupunkt, med Googles begivenheder lagt over egne opgaver

Samme gitter som Kommende, givet en liste mere at tegne. Ikke et gitter nummer
to, og grunden er den del, der er nem at få næsten rigtig: et mål skal afgøre
ud fra `dataTransfer.types` alene, om det tager imod, og to kopier af det er to
steder at være næsten rigtig. En opgave trukket her flyttes præcis som alle
andre steder, fordi det er den samme kode, der gør det.

En begivenhed er ikke en opgave, og cellen siger det før et eneste ord er
læst: en flade af farve, hvor en opgave er et kort med en farvet kant. Den kan
ikke trækkes — forbindelsen er læsende, og en chip, der letter fra siden og så
ikke vil lande, læses som en fejl frem for som en regel — og den er ikke en
knap, for der er ikke noget at lukke ved en andens møde. Farven er Googles egen
for den kalender, så to kalendere i ét gitter kendes fra hinanden på den farve,
personen allerede kender dem på.

Farven tjekkes frem for at blive troet: den ender i en inline `style`, og en
værdi, der ikke er en farve, kan lukke erklæringen og åbne en anden. Linket
tegnes kun, hvis det begynder med `https://` — et href er dér, hvor en
`javascript:`-adresse bliver til et klik, der kører.

Prøven bruger uge-visningen med vilje: en månedscelle skæres af ved tre chips,
og resten af suiten lægger opgaver på andre dage. Datoen læses ud af siden, ikke
ud af testprocessen, som de øvrige har lært at gøre.

web/src/lib/components/CalendarView.svelte

@@ -49,7 +49,19 @@
 		 * flight swallows the click that would page. A week that straddles the
 		 * 31st and the 1st has both days in the same row.
 		 */
-		span = 'month'
+		span = 'month',
+		/**
+		 * Events read from somewhere else, laid over the same grid.
+		 *
+		 * An event is not a task, and the cell has to say so. It has a span of time
+		 * rather than a day, it cannot be ticked off, and — while the connection is
+		 * read-only — it cannot be moved: verdande holds a copy of somebody else's
+		 * calendar, and a chip that lets itself be dragged is a promise the server
+		 * has no way to keep.
+		 *
+		 * Empty everywhere but Kalender, so no other view pays for it.
+		 */
+		events = []
 	} = $props();
 
 	let cursor = $state(span === 'week' ? startOfWeek(new Date()) : startOfMonth(new Date()));
@@ -137,6 +149,53 @@
 				(!projectId || t.project_id === projectId)
 		);
 
+	// Which cells an event covers, rather than where it starts. A trip from Friday
+	// to Monday is in all four, and asking only about the start day would draw it
+	// once and leave three days looking clear.
+	const eventsOn = (date) => {
+		const day = iso(date);
+		return events.filter((e) => e.start_day <= day && e.end_day >= day);
+	};
+
+	/**
+	 * The time on the chip, and only where it means something.
+	 *
+	 * An all-day event has no time. A timed one shows its start, and only on the
+	 * day it starts: a meeting that runs past midnight would otherwise claim to
+	 * begin at 20:00 on both days.
+	 *
+	 * The clock is read out of the string rather than parsed into a Date. Google
+	 * writes the event's own offset into it, and handing that to the browser asks
+	 * *this device* what time it is — so a phone in another time zone would show a
+	 * Copenhagen meeting an hour out.
+	 */
+	function timeOn(event, date) {
+		if (event.all_day || !event.starts_at) return '';
+		if (event.start_day !== iso(date)) return '';
+		return event.starts_at.slice(11, 16);
+	}
+
+	/**
+	 * The calendar's own colour, or nothing.
+	 *
+	 * Checked rather than trusted, even though it comes from Google: this goes into
+	 * an inline `style`, and a value that is not a colour is a value that can close
+	 * the declaration and open another. A hex triple is the whole of what Google
+	 * sends, so anything else is not a colour that failed to render — it is a
+	 * string that has no business being here.
+	 */
+	const HEX = /^#[0-9a-f]{3,8}$/i;
+	const swatch = (colour) => (HEX.test(colour ?? '') ? colour : '');
+
+	/**
+	 * Whether the event links back to Google.
+	 *
+	 * `https:` and nothing else. An href is where a `javascript:` URL becomes a
+	 * click that runs, and the address in this field was written by a service, not
+	 * by the person looking at it.
+	 */
+	const linkOf = (event) => (String(event.url ?? '').startsWith('https://') ? event.url : '');
+
 	const monthName = $derived(
 		cursor.toLocaleDateString(tag(), { month: 'long', year: 'numeric' })
 	);
@@ -232,7 +291,14 @@
 
 	<div class="grid">
 		{#each grid as date (date.toISOString())}
-			{@const tasks = tasksOn(date)}
+			{@const dayEvents = eventsOn(date)}
+			<!-- Events first, then whatever room is left goes to tasks. An event is
+			     fixed to a time somebody else chose; a task is only dated, and if
+			     something in a full cell has to fall behind "+3 mere", it should be
+			     the one that can be moved. -->
+			{@const tasks = tasksOn(date).slice(0, Math.max(0, chipLimit - dayEvents.length))}
+			{@const hidden =
+				tasksOn(date).length - tasks.length + Math.max(0, dayEvents.length - chipLimit)}
 			<!-- A week has no outside: every day in it is the week you asked for. -->
 			{@const outside = span !== 'week' && date.getMonth() !== cursor.getMonth()}
 			<!-- svelte-ignore a11y_no_static_element_interactions -->
@@ -254,7 +320,36 @@
 				{/if}
 				<span class="number">{date.getDate()}</span>
 
-				{#each tasks.slice(0, chipLimit) as task (task.id)}
+				{#each dayEvents.slice(0, chipLimit) as event (event.id)}
+					{@const link = linkOf(event)}
+					{@const label = t('cal.eventCannotMove', {
+						name: event.summary,
+						calendar: event.calendar_name || t('cal.title')
+					})}
+					<!-- Not draggable, and it says so twice: the attribute stops the
+					     browser dragging an anchor of its own accord, and the title says
+					     why to anybody who tries. The drop targets would refuse it anyway
+					     — they only accept the task MIME type — but a chip that lifts off
+					     the page and then will not land reads as a bug rather than as a
+					     rule. It is also not a button: there is nothing here to complete,
+					     and a checkbox next to a meeting is an offer verdande cannot keep. -->
+					<svelte:element
+						this={link ? 'a' : 'span'}
+						class="event"
+						class:allday={event.all_day}
+						draggable="false"
+						href={link || undefined}
+						target={link ? '_blank' : undefined}
+						rel={link ? 'noreferrer noopener' : undefined}
+						title={link ? `${label} — ${t('cal.openInGoogle')}` : label}
+						style={swatch(event.colour) ? `--event-colour: ${swatch(event.colour)}` : undefined}
+					>
+						{#if timeOn(event, date)}<span class="at">{timeOn(event, date)}</span>{/if}
+						{event.summary}
+					</svelte:element>
+				{/each}
+
+				{#each tasks as task (task.id)}
 					<!-- Draggable as well as clickable: within a month, moving something
 					     to another day is the whole reason to be looking at a month. The
 					     browser needs a few pixels of movement before it calls a press a
@@ -271,8 +366,8 @@
 					</button>
 				{/each}
 
-				{#if tasks.length > chipLimit}
-					<span class="more">{t('view.more', { n: tasks.length - chipLimit })}</span>
+				{#if hidden > 0}
+					<span class="more">{t('view.more', { n: hidden })}</span>
 				{/if}
 			</div>
 		{/each}
@@ -451,6 +546,58 @@
 		background: var(--surface);
 	}
 
+	/* An event reads as a different kind of thing at a glance, before any of the
+	   words are read: a solid bar of colour where a task is a card with a coloured
+	   edge. That is the whole distinction the eye needs — a task is something you
+	   can move and finish, an event is something that is simply happening.
+
+	   The colour is Google's own for that calendar, so two calendars in one grid
+	   are told apart by the colour the person already knows them by. `--line-strong`
+	   is the fallback for a calendar Google gave no colour, which is rare and not
+	   worth a second decision. */
+	.event {
+		--event-colour: var(--line-strong);
+		font-size: var(--text-xs);
+		text-align: left;
+		display: block;
+		padding: 1px var(--s1);
+		border-radius: var(--radius-sm);
+		border-left: 3px solid var(--event-colour);
+		background: var(--surface-sunken);
+		color: var(--ink-muted);
+		text-decoration: none;
+		overflow: hidden;
+		text-overflow: ellipsis;
+		white-space: nowrap;
+		/* A drag that cannot land is worse than one that cannot start. The grab
+		   cursor is the promise; this withholds it. */
+		cursor: default;
+	}
+
+	.event[href] {
+		cursor: pointer;
+	}
+
+	.event:hover {
+		background: var(--surface);
+		color: var(--ink);
+	}
+
+	/* An all-day event has no clock to show, so the colour carries it instead:
+	   filled rather than merely edged, which is what an all-day band looks like in
+	   every calendar anybody has used. */
+	.event.allday {
+		border-left: none;
+		background: color-mix(in oklab, var(--event-colour) 22%, var(--ground));
+		box-shadow: inset 0 0 0 1px color-mix(in oklab, var(--event-colour) 45%, transparent);
+	}
+
+	.at {
+		font-variant-numeric: tabular-nums;
+		color: var(--ink-faint);
+		margin-right: 2px;
+	}
+
 	.more {
 		font-size: var(--text-xs);
 		color: var(--ink-faint);
@@ -482,6 +629,29 @@
 			background: var(--p3);
 		}
 
+		/* Counted rather than read, like the task chips beside it — a 45px column
+		   fits no title. A bar rather than a dot, so an event is still tellable from
+		   a task at a glance: what the month grid says on a phone is "how much is
+		   on this day, and how much of it is somebody else's". */
+		.event {
+			font-size: 0;
+			padding: 0;
+			height: 4px;
+			border-radius: 1px;
+			border-left: none;
+			box-shadow: none;
+			background: var(--event-colour);
+		}
+
+		.event.allday {
+			background: var(--event-colour);
+			box-shadow: none;
+		}
+
+		.at {
+			display: none;
+		}
+
 		.more {
 			font-size: 10px;
 		}
@@ -536,6 +706,27 @@
 			background: var(--surface-raised);
 		}
 
+		/* Readable again in the week, for the reason the task chips are: a
+		   full-width row has room for a title, and a week somebody opened on
+		   purpose is one they want to read rather than count. */
+		.calendar.week .event {
+			font-size: var(--text-xs);
+			height: auto;
+			padding: 1px var(--s1);
+			border-radius: var(--radius-sm);
+			border-left: 3px solid var(--event-colour);
+			background: var(--surface-sunken);
+		}
+
+		.calendar.week .event.allday {
+			border-left: none;
+			background: color-mix(in oklab, var(--event-colour) 22%, var(--ground));
+		}
+
+		.calendar.week .at {
+			display: inline;
+		}
+
 		.calendar.week .more {
 			font-size: var(--text-xs);
 		}

web/src/lib/components/Sidebar.svelte

@@ -145,7 +145,16 @@
 			// Sidst i rækken, fordi `navOrder` føjer en ukendt nøgle til bagest: står
 			// den et andet sted her, ser en ny konto én rækkefølge og alle
 			// eksisterende en anden.
-			{ key: 'notes', href: '/noter', label: 'notes.title' }
+			{ key: 'notes', href: '/noter', label: 'notes.title' },
+			// Kalenderen bærer et gitter, af samme grund som Noter bærer et ark: den
+			// er ikke endnu et filter over de samme opgaver. Den er de samme opgaver
+			// med noget andet lagt over — Googles begivenheder — og en prik ville
+			// sige, at den hørte til blandt I dag og Kommende.
+			//
+			// Sidst i rækken, fordi `navOrder` føjer en ukendt nøgle til bagest: står
+			// den et andet sted her, ser en ny konto én rækkefølge og alle
+			// eksisterende en anden.
+			{ key: 'calendar', href: '/kalender', label: 'nav.calendar' }
 		].filter(Boolean)
 	);
 
@@ -488,6 +497,26 @@
 								/>
 								<path d="M9 1.7V5h3.3" fill="none" stroke="currentColor" stroke-width="1.4" />
 							</svg>
+						{:else if item.key === 'calendar'}
+							<svg class="mark" viewBox="0 0 16 16" aria-hidden="true">
+								<rect
+									x="2.2"
+									y="3.2"
+									width="11.6"
+									height="10.6"
+									rx="1.4"
+									fill="none"
+									stroke="currentColor"
+									stroke-width="1.4"
+								/>
+								<path
+									d="M2.2 6.4h11.6M5.6 2.2v2M10.4 2.2v2"
+									fill="none"
+									stroke="currentColor"
+									stroke-width="1.4"
+									stroke-linecap="round"
+								/>
+							</svg>
 						{:else}
 							<span class="dot" class:today={item.key === 'today'} aria-hidden="true"></span>
 						{/if}

web/src/lib/stores.svelte.js

@@ -835,6 +835,43 @@ function read() {
 
 export const upcomingView = new UpcomingView();
 
+/**
+ * Month or week on Kalender.
+ *
+ * Its own key rather than Kommende's, even though the two switchers look alike.
+ * They are not the same choice: Kommende opens on a list of the next seven days
+ * and offers a grid, and Kalender is the grid — sharing the value would mean
+ * choosing a week on one and finding the other had changed, and one of them would
+ * have to answer for the `list` mode the other does not have.
+ *
+ * In localStorage rather than on the account, and for the same reason as the
+ * sidebar's width: a grid of seven columns needs room, and the answer on a phone
+ * is not the answer on a wide monitor.
+ */
+const CALENDAR_MODES = ['week', 'month'];
+
+class CalendarViewMode {
+	mode = $state(readCalendarMode());
+
+	set(next) {
+		if (!CALENDAR_MODES.includes(next)) return;
+		this.mode = next;
+		try {
+			localStorage.setItem('verdande:calendar', next);
+		} catch {
+			// Private browsing; the choice simply will not persist.
+		}
+	}
+}
+
+function readCalendarMode() {
+	if (typeof localStorage === 'undefined') return 'month';
+	const stored = localStorage.getItem('verdande:calendar');
+	return CALENDAR_MODES.includes(stored) ? stored : 'month';
+}
+
+export const calendarView = new CalendarViewMode();
+
 /**
  * Whether the lists show what has already been finished.
  *