feat: add WebDAV file browser with auth improvements

Add a new Files section powered by WebDAV for browsing, uploading,
downloading, renaming, and deleting files and folders.

New features:
- WebDAV file browser with grid/list views and breadcrumb navigation
- File upload (drag-and-drop and button), folder creation, rename, delete
- File preview modals for images and other file types
- WebDAV proxy API route to handle authentication
- Navigation rail entry for Files (auto-hidden when WebDAV is unsupported)

Auth improvements:
- Fix premature redirects on calendar, contacts, and settings pages by
  adding explicit auth check on mount before redirecting to login
- Persist active settings tab in localStorage

Other:
- Expose getAuthHeader() and getServerUrl() on JMAPClient
- Add WebDAV store with connection testing and capability detection
- Add i18n translations for file browser in all 8 locales (de, en, es,
  fr, it, ja, nl, pt)

locales/nl/common.json

@@ -61,6 +61,7 @@
     "contacts": "Contacten",
     "calendar": "Agenda",
     "settings": "Instellingen",
+    "files": "Bestanden",
     "loading_mailboxes": "Mappen laden...",
     "push_connected": "Real-time updates actief",
     "push_disconnected": "Real-time updates inactief",
@@ -1692,5 +1693,83 @@
     "got_it": "Begrepen",
     "settings": "Instellingen",
     "dismiss": "Sluiten"
+  },
+  "files": {
+    "title": "Bestanden",
+    "search_placeholder": "Bestanden zoeken...",
+    "empty_state_title": "Nog geen bestanden",
+    "empty_state_description": "Upload bestanden of maak mappen aan om te beginnen",
+    "upload": "Uploaden",
+    "upload_files": "Bestanden uploaden",
+    "new_folder": "Nieuwe map",
+    "new_folder_name": "Mapnaam",
+    "rename": "Hernoemen",
+    "rename_title": "Hernoemen",
+    "new_name": "Nieuwe naam",
+    "delete": "Verwijderen",
+    "delete_confirm_title": "Bron verwijderen",
+    "delete_confirm_message": "Weet u zeker dat u \"{name}\" wilt verwijderen? Dit kan niet ongedaan worden gemaakt.",
+    "download": "Downloaden",
+    "name": "Naam",
+    "size": "Grootte",
+    "modified": "Gewijzigd",
+    "type": "Type",
+    "folder": "Map",
+    "file": "Bestand",
+    "parent_directory": "Bovenliggende map",
+    "breadcrumb_root": "Start",
+    "drop_files_here": "Sleep bestanden hierheen om te uploaden",
+    "uploading": "Uploaden...",
+    "upload_success": "{count, plural, one {1 bestand geüpload} other {# bestanden geüpload}}",
+    "upload_error": "Bestand uploaden mislukt",
+    "create_folder_success": "Map aangemaakt",
+    "create_folder_error": "Map aanmaken mislukt",
+    "delete_success": "Succesvol verwijderd",
+    "delete_error": "Verwijderen mislukt",
+    "rename_success": "Succesvol hernoemd",
+    "rename_error": "Hernoemen mislukt",
+    "download_error": "Downloaden mislukt",
+    "not_available": "WebDAV-bestandsopslag is niet beschikbaar op deze server",
+    "cancel": "Annuleren",
+    "create": "Aanmaken",
+    "save": "Opslaan",
+    "no_results": "Geen bestanden komen overeen met uw zoekopdracht",
+    "batch_delete_confirm_message": "Weet u zeker dat u {count, plural, one {1 item} other {# items}} wilt verwijderen? Dit kan niet ongedaan worden gemaakt.",
+    "batch_delete_success": "{count, plural, one {1 item verwijderd} other {# items verwijderd}}",
+    "grid_view": "Rasterweergave",
+    "list_view": "Lijstweergave",
+    "details": "Details",
+    "path": "Pad",
+    "preview": "Voorbeeld",
+    "preview_error": "Voorbeeld laden mislukt",
+    "cut": "Knippen",
+    "copy": "Kopiëren",
+    "paste": "Plakken",
+    "move_success": "{count, plural, one {1 item verplaatst} other {# items verplaatst}}",
+    "move_error": "Verplaatsen mislukt",
+    "paste_success": "Succesvol geplakt",
+    "paste_error": "Plakken mislukt",
+    "new_text_file": "Nieuw tekstbestand",
+    "file_name": "Bestandsnaam",
+    "retry": "Opnieuw proberen",
+    "refresh": "Vernieuwen",
+    "toggle_favorite": "Favoriet aan/uit",
+    "duplicate": "Dupliceren",
+    "duplicate_success": "Succesvol gedupliceerd",
+    "duplicate_error": "Dupliceren mislukt",
+    "create_file_success": "Bestand aangemaakt",
+    "create_file_error": "Bestand aanmaken mislukt",
+    "favorites": "Favorieten",
+    "recent": "Recent",
+    "properties": "Eigenschappen",
+    "open_folder": "Map openen",
+    "upload_folder": "Map uploaden",
+    "file_too_large": "\"{name}\" overschrijdt de maximale bestandsgrootte ({max})",
+    "undo": "Ongedaan maken",
+    "undo_success": "Actie ongedaan gemaakt",
+    "undo_error": "Ongedaan maken mislukt",
+    "toolbar": "Bestandsacties",
+    "file_list": "Bestanden en mappen",
+    "context_menu": "Acties"
   }
 }