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/push.js

@@ -8,6 +8,7 @@
  */
 
 import { api } from './api.js';
+import { t } from './i18n.svelte.js';
 
 /** Whether this browser can do it at all. Safari before 16.4 cannot, and neither
  *  can any browser on a page served over plain HTTP from a non-localhost host. */
@@ -56,14 +57,14 @@ export async function subscribe() {
 	if (granted !== 'granted') {
 		throw new Error(
 			granted === 'denied'
-				? 'Browseren har blokeret notifikationer for dette site. Det skal slås til i browserens indstillinger.'
-				: 'Notifikationer blev ikke slået til.'
+				? t('push.deniedByBrowser')
+				: t('push.notEnabled')
 		);
 	}
 
 	const reg = await registration();
 	const { public_key } = await api.pushKey();
-	if (!public_key) throw new Error('Serveren har ingen VAPID-nøgle.');
+	if (!public_key) throw new Error(t('push.noVapidKey'));
 
 	const subscription = await reg.pushManager.subscribe({
 		// Not optional in Chrome: a subscription that is not userVisibleOnly is