Web: eight more UI languages behind a single language registry

Adds Swiss German, French, Italian, Dutch, Danish, Norwegian, Swedish and
Finnish — ten selectable languages, each complete at 418/418 keys with
matching placeholders and date arrays.

A language used to be registered in four unrelated places (dictionary,
settings dropdown, date-picker locale ternary, resolver fallback), so
adding one meant touching all of them. LANGUAGES in i18n.js is now the
single source of truth: adding a language is one entry plus one
dictionary, and the dropdown derives from it.

The registry also separates the stored code from the BCP-47 tag used for
formatting. That distinction is what makes 'ch' safe — "ch" is a country,
not a language, and would throw if handed to Intl; it maps to de-CH.

Fixes locale handling along the way: fmtTime/fmtDatetime in calendar.js,
month.js and week.js hardcoded 'de', so English users saw German-formatted
times, and two toLocaleDateString() calls silently used the browser locale
instead of the app language. All formatting now goes through getLocale().
Missing keys fall back language → English → German rather than straight to
German, and the dead per-dictionary `locale` key is gone.

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

frontend/js/settings-sync.js

@@ -5,6 +5,8 @@
 // syncable value so that "not synced" works per browser, plus the declarative
 // table definition the settings UI renders from. See backend/SETTINGS_SYNC.md.
 
+import { LANGUAGES } from './i18n.js';
+
 // Canonical default colours — the single source for the web. Reset writes these.
 export const DEFAULT_COLORS = {
   primary_color:       '#58B900',
@@ -59,9 +61,11 @@ export const SETTING_GROUPS = [
   {
     titleKey: 'settings_language',
     rows: [
-      { key: 'language', labelKey: 'settings_language', type: 'select', opts: [
-        { v: 'de', label: 'Deutsch' }, { v: 'en', label: 'English' },
-      ] },
+      // Derived from the i18n registry: adding a language there adds it here.
+      // Each language is listed under its own endonym, so the labels are never
+      // translated — a Finn looks for "Suomi", not "Finnisch".
+      { key: 'language', labelKey: 'settings_language', type: 'select',
+        opts: LANGUAGES.map(l => ({ v: l.code, label: l.label })) },
       { key: 'share_calendar_icon', labelKey: 'settings_share_icon', type: 'icon' },
     ],
   },