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/css/style.css

@@ -3584,6 +3584,35 @@ a:hover {
     transition: background 0.2s, border-color 0.2s;
 }
 
+/* === Language Switcher === */
+.lang-switcher {
+    display: flex;
+    gap: 2px;
+    background: var(--bg-secondary);
+    border-radius: var(--radius);
+    padding: 2px;
+}
+.lang-btn {
+    padding: 4px 8px;
+    border-radius: var(--radius);
+    font-size: 11px;
+    font-weight: 600;
+    letter-spacing: 0.5px;
+    cursor: pointer;
+    background: transparent;
+    color: var(--text-secondary);
+    border: none;
+    transition: background 0.15s, color 0.15s;
+}
+.lang-btn:hover {
+    color: var(--text-primary);
+    background: var(--bg-hover);
+}
+.lang-btn.active {
+    background: var(--accent);
+    color: var(--bg-primary);
+}
+
 /* === Light Theme Sonderregeln === */
 [data-theme="light"] .sidebar {
     border-right: 1px solid var(--border);