i18n: Complete DE/EN language switcher integration

- Add LangManager with 270+ translation keys, anti-flicker lang detection
- Replace all hardcoded German strings in app.js, components.js, dashboard.html, index.html
- Dynamic getter properties for fact-check labels, category badges
- Language-aware map tiles (DE/EN OSM servers), CSP updated for tile.openstreetmap.org
- Lang switcher in header bar and login page
- Locale-aware date formatting, translateApiError for backend messages

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/static/js/lang.js

@@ -335,6 +335,16 @@ const TRANSLATIONS = {
   'sources.feeds_count':            { de: '{n} Feeds',                                                     en: '{n} Feeds' },
   'sources.feed_count':             { de: '{n} Feed',                                                      en: '{n} Feed' },
   'sources.articles_from_sources':  { de: '{articles} Artikel aus {sources} Quellen',                      en: '{articles} articles from {sources} sources' },
+  'sources.cat_short_nachrichtenagentur': { de: 'Agentur',                                                   en: 'Agency' },
+  'sources.cat_short_oeffentlich_rechtlich': { de: 'ÖR',                                                     en: 'Public' },
+  'sources.cat_short_qualitaetszeitung': { de: 'Qualität',                                                   en: 'Quality' },
+  'sources.cat_short_behoerde':    { de: 'Behörde',                                                          en: 'Gov' },
+  'sources.cat_short_fachmedien':  { de: 'Fach',                                                             en: 'Trade' },
+  'sources.cat_short_think_tank':  { de: 'Think Tank',                                                       en: 'Think Tank' },
+  'sources.cat_short_international': { de: 'Intl.',                                                          en: 'Intl.' },
+  'sources.cat_short_regional':    { de: 'Regional',                                                         en: 'Regional' },
+  'sources.cat_short_boulevard':   { de: 'Boulevard',                                                        en: 'Tabloid' },
+  'sources.cat_short_sonstige':    { de: 'Sonstige',                                                         en: 'Other' },
 
   // ── Sidebar labels ─────────────────────────────────────
   'sidebar.no_adhoc':          { de: 'Keine Ad-hoc-Lagen',                                                 en: 'No ad-hoc incidents' },
@@ -494,6 +504,11 @@ const LangManager = (() => {
       const key = el.getAttribute('data-i18n-title');
       if (key) el.setAttribute('title', t(key));
     });
+
+    // lang-switcher active state
+    document.querySelectorAll('.lang-btn').forEach(btn => {
+      btn.classList.toggle('active', btn.getAttribute('data-lang') === _lang);
+    });
   }
 
   /**