feat(5.2): add User Preferences API with full-stack implementation

Backend:
- Create app/models/user_preference.py with TenantMixin (user_id, key, value JSONB)
- Create app/routes/user_preferences.py with GET/PUT/DELETE endpoints + RBAC
- Add user_preferences:read/write to CORE_PERMISSIONS
- Add user_preferences to legacy role permissions (admin/editor/viewer)
- Register route in app/main.py and app/routes/__init__.py
- Create alembic migration 0028_user_preferences
- Add UserPreference model to conftest.py for test schema
- Fix pre-existing conftest seed (Contact industry field removed in migration 0027)

Frontend:
- Create frontend/src/api/userPreferences.ts with React Query hooks
- Create frontend/src/hooks/useUserPreferences.ts syncing with uiStore
- Add i18n entries for de.json and en.json

Tests:
- 13 tests covering CRUD, tenant isolation, CSRF, unauthenticated access
- All tests passing

frontend/src/i18n/locales/de.json

@@ -893,6 +893,20 @@
     "changes": "Änderungen",
     "fieldsChanged": "Felder geändert"
   },
+  "userPreferences": {
+    "title": "Benutzereinstellungen",
+    "saved": "Einstellungen gespeichert",
+    "saveError": "Fehler beim Speichern der Einstellungen",
+    "deleted": "Einstellung gelöscht",
+    "deleteError": "Fehler beim Löschen der Einstellung",
+    "loading": "Einstellungen werden geladen",
+    "theme": "Design",
+    "language": "Sprache",
+    "sidebar": "Seitenleiste",
+    "activeTab": "Aktiver Tab",
+    "sortOrder": "Sortierreihenfolge",
+    "synced": "Einstellungen synchronisiert"
+  },
   "ai": {
     "uiControl": {
       "active": "KI steuert die UI",