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/lib/components/Sidebar.svelte

@@ -328,7 +328,7 @@
 	}
 </script>
 
-<nav class="sidebar" aria-label="Hovedmenu">
+<nav class="sidebar" aria-label={t('nav.main')}>
 	<div class="brand">
 		<!-- Verdande's mark: the rune Wunjo, which is the letter the Norn's name
 		     starts with in the elder futhark. One glyph, no wordmark beside it —
@@ -573,8 +573,8 @@
 									class="swatch"
 									class:chosen={(group.color ?? 'graphite') === color.id}
 									style="background: {colorVar(color.id)}"
-									title={color.name}
-									aria-label={color.name}
+									title={t(color.name)}
+									aria-label={t(color.name)}
 									aria-pressed={(group.color ?? 'graphite') === color.id}
 									onmousedown={(e) => e.preventDefault()}
 									onclick={() => app.setGroupColor(group.id, color.id)}