feat(i18n): add internationalization framework with 10 languages

- Create i18n utility module (app/utils/i18n.py) with translation loading,
  browser language detection, AI fallback, and l10n helpers
- Add JSON translation files for EN, DE, FR, ES, IT, PT, NL, PL, ZH, RU
- Add preferred_language column to UserProfile model with migration
- Register _() translation function as Jinja2 global
- Update base.html with translated navigation, footer, cookie notice
- Add language selector dropdown in nav bar (desktop + mobile)
- Create API endpoints for language preference (POST/GET /api/i18n/)
- Support language detection: user profile > cookie > Accept-Language > default

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

app/api/__init__.py

@@ -17,6 +17,7 @@ from app.api.dropbox import router as dropbox_router
 from app.api.duplicates import router as duplicates_router
 from app.api.files import router as files_router
 from app.api.google_drive import router as google_drive_router
+from app.api.i18n import router as i18n_router
 from app.api.imap_accounts import router as imap_accounts_router
 from app.api.integrations import router as integrations_router
 from app.api.logs import router as logs_router
@@ -82,3 +83,4 @@ router.include_router(imap_accounts_router)
 router.include_router(integrations_router)
 router.include_router(notifications_router)
 router.include_router(scheduled_jobs_router)
+router.include_router(i18n_router)