feat(i18n): Export-Modal + Quellenverwaltung + Chat-Widget + Stats-Bar

- Export-Modal: Titel, Bereiche, Format, alle Checkboxes (Zusammenfassung,
  Recherchebericht / Lagebild, Faktencheck, Quellen), PDF/DOCX, Abbrechen,
  Exportieren.
- Quellenverwaltung-Modal: Title, 8 Filter-Labels (sr-only) + 8 Alle-*
  Default-Optionen, Search-Placeholder + Label, + Quelle-Button, Add-
  Form (URL/Erkennen/Name/Kategorie/Typ/RSS-URL/Domain/Notizen +
  Placeholder), Speichern/Abbrechen, Loading-State.
- Stats-Bar (app.js): RSS-Feeds/Web-Quellen/Ausgeschlossen-Labels.
- components.js: source-excluded-badge.
- Chat-Widget: Title, alle 5 Buttons mit title+aria, Input-Placeholder.
- Chat-Begruessung in chat.js auf T() umgestellt.
- 50+ neue i18n-Keys. Cache-Buster components.js + chat.js + app.js
  auf v=20260514e gebumpt.

src/static/js/app.js

@@ -2778,10 +2778,10 @@ async handleRefresh() {
         const excluded = this._myExclusions.length;
 
         bar.innerHTML = `
-            <span class="sources-stat-item"><span class="sources-stat-value">${rss.count}</span> RSS-Feeds</span>
-            <span class="sources-stat-item"><span class="sources-stat-value">${web.count}</span> Web-Quellen</span>
+            <span class="sources-stat-item"><span class="sources-stat-value">${rss.count}</span> ${(typeof T === 'function' ? T('sources_modal.stats.rss', 'RSS-Feeds') : 'RSS-Feeds')}</span>
+            <span class="sources-stat-item"><span class="sources-stat-value">${web.count}</span> ${(typeof T === 'function' ? T('sources_modal.stats.web', 'Web-Quellen') : 'Web-Quellen')}</span>
             <span class="sources-stat-item"><span class="sources-stat-value">${tg.count}</span> Telegram</span>
-            <span class="sources-stat-item"><span class="sources-stat-value">${excluded}</span> Ausgeschlossen</span>
+            <span class="sources-stat-item"><span class="sources-stat-value">${excluded}</span> ${(typeof T === 'function' ? T('sources_modal.stats.excluded', 'Ausgeschlossen') : 'Ausgeschlossen')}</span>
             <span class="sources-stat-item"><span class="sources-stat-value">${stats.total_articles}</span> Artikel gesamt</span>
         `;
     },