feat(i18n): the interface speaks both languages

The account has had a `locale` since the beginning and it only chose which grammar
quick add read a line with — so picking English changed nothing anybody could see,
and the field said so honestly: "Sprog i hurtig tilføjelse". It says "Sprog" now,
because it means it.

Every component goes through `t()`. Danish is the source: a key missing from
English falls through to Danish rather than to the key, because a Danish sentence
in an English screen can be read and `settings.notifications.title` cannot.

`tag()` is the single place `Intl` is asked. Before, `'da-DK'` was written into a
dozen components, and that is the half of a translation that gets left behind — an
English interface still saying "24. aug." is exactly the shape of a job half done.
There is a smoke test that switches to English and refuses to see a Danish month
name in the calendar's heading.

Colour and theme names are keys now. `colors.js` keeps its ids untouched — they
are what the database stores, and `internal/store` reads them out of that file —
but `name` is `'color.tomato'`, resolved where it is drawn.

The Inbox keeps its name. It is a project row the server creates at signup, not a
string in the interface, and it can be renamed like any other project.

Four copies of "3 hours ago" became one, in `web/src/lib/when.js`. They had
already drifted: one said "aldrig logget ind", another "aldrig", and two stopped
at days where the others carried on to a date.

Routes stay Danish. `/indstillinger`, `/projekt`, `/uddelegeret` are bookmarks and
they are in every smoke test; translating them buys nothing and costs both.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web/src/routes/etiket/[navn]/+page.svelte

@@ -10,6 +10,7 @@
 	import { api } from '$lib/api.js';
 	import { app } from '$lib/stores.svelte.js';
 	import TaskRow from '$lib/components/TaskRow.svelte';
+	import { t } from '$lib/i18n.svelte.js';
 
 	let loaded = $state(false);
 	let name = $derived(decodeURIComponent($page.params.navn ?? ''));
@@ -42,7 +43,7 @@
 	{#if loaded && !open.length}
 		<p class="clear">
 			<span class="rune" aria-hidden="true">ᚹ</span>
-			Ingen åbne opgaver med denne etiket.
+			{t('view.labelEmpty')}
 		</p>
 	{/if}
 </div>