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/date-picker.js

@@ -5,7 +5,7 @@
  *   value  : ISO string ("YYYY-MM-DDTHH:MM" | "YYYY-MM-DD") or ""
  *   mode   : 'datetime' | 'date'
  */
-import { t } from './i18n.js';
+import { t, getLocale } from './i18n.js';
 
 const ITEM_H  = 40;  // px per scroll item
 const VISIBLE = 3;   // visible items in time scroller
@@ -250,16 +250,16 @@ export function openDatePicker(anchor, value, mode = 'datetime') {
 /**
  * Format an ISO value for display in the UI
  * mode: 'datetime' | 'date'
- * lang: 'de' | 'en'
+ * Formatting follows the active UI language via the i18n locale registry.
  */
-export function formatDtDisplay(isoStr, mode, lang = 'de') {
+export function formatDtDisplay(isoStr, mode) {
   if (!isoStr) return '—';
   try {
     const d = mode === 'datetime'
       ? new Date(isoStr.replace(' ', 'T'))
       : new Date(isoStr + 'T00:00:00');
     if (isNaN(d)) return isoStr;
-    const locale = lang === 'en' ? 'en-GB' : 'de-CH';
+    const locale = getLocale();
     if (mode === 'datetime') {
       return d.toLocaleString(locale, {
         day: '2-digit', month: '2-digit', year: 'numeric',