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/main.py

@@ -235,7 +235,7 @@ class SecurityHeadersMiddleware(BaseHTTPMiddleware):
             "script-src 'self' 'unsafe-inline' https://cdn.jsdelivr.net; "
             "style-src 'self' 'unsafe-inline' https://fonts.googleapis.com https://cdn.jsdelivr.net; "
             "font-src 'self' https://fonts.gstatic.com; "
-            "img-src 'self' data: https://tile.openstreetmap.de; "
+            "img-src 'self' data: https://tile.openstreetmap.de https://tile.openstreetmap.org; "
             "connect-src 'self' wss: ws:; "
             "frame-ancestors 'none'"
         )