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/messages/en.ts

@@ -5,6 +5,7 @@ export const dict = {
   'common.loading': 'Loading...',
   'common.unavailable': 'Unavailable',
   'common.language.english': 'English',
+  'common.language.german': 'German',
   'common.language.french': 'French',
   'common.language.simplifiedChinese': 'Chinese (Simplified)',
   'common.language.traditionalChinese': 'Chinese (Traditional)',