feat(i18n): add German (de) locale

Adds a complete German translation covering all 4 735 UI strings.

New files:
- packages/ui/src/lib/i18n/messages/de.ts
- packages/ui/src/lib/i18n/messages/de.settings.ts

Modified files:
- runtime.ts: add 'de' to Locale type, LOCALES array, LOCALE_LABEL_KEYS
- store.ts: add 'de' case in loadDictionary
- messages/en.ts: add common.language.german key

packages/ui/src/lib/i18n/store.ts

@@ -42,7 +42,9 @@ async function loadDictionary(locale: Locale): Promise<I18nDictionary> {
               ? await import('./messages/ko') as { dict: I18nDictionary }
               : locale === 'pl'
                 ? await import('./messages/pl') as { dict: I18nDictionary }
-                : locale === 'ja'
+                : locale === 'de'
+      ? await import('./messages/de') as { dict: I18nDictionary }
+      : locale === 'ja'
                   ? await import('./messages/ja') as { dict: I18nDictionary }
                   : { dict: enDict };
   dictionaries.set(locale, mod.dict);