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/en/common.json

@@ -61,6 +61,7 @@
     "contacts": "Contacts",
     "calendar": "Calendar",
     "settings": "Settings",
+    "files": "Files",
     "loading_mailboxes": "Loading mailboxes...",
     "push_connected": "Real-time updates active",
     "push_disconnected": "Real-time updates inactive",
@@ -1729,5 +1730,83 @@
     "got_it": "Got it",
     "settings": "Settings",
     "dismiss": "Dismiss"
+  },
+  "files": {
+    "title": "Files",
+    "search_placeholder": "Search files...",
+    "empty_state_title": "No files yet",
+    "empty_state_description": "Upload files or create folders to get started",
+    "upload": "Upload",
+    "upload_files": "Upload Files",
+    "new_folder": "New Folder",
+    "new_folder_name": "Folder name",
+    "rename": "Rename",
+    "rename_title": "Rename",
+    "new_name": "New name",
+    "delete": "Delete",
+    "delete_confirm_title": "Delete resource",
+    "delete_confirm_message": "Are you sure you want to delete \"{name}\"? This cannot be undone.",
+    "download": "Download",
+    "name": "Name",
+    "size": "Size",
+    "modified": "Modified",
+    "type": "Type",
+    "folder": "Folder",
+    "file": "File",
+    "parent_directory": "Parent directory",
+    "breadcrumb_root": "Home",
+    "drop_files_here": "Drop files here to upload",
+    "uploading": "Uploading...",
+    "upload_success": "{count, plural, one {1 file uploaded} other {# files uploaded}}",
+    "upload_error": "Failed to upload file",
+    "create_folder_success": "Folder created",
+    "create_folder_error": "Failed to create folder",
+    "delete_success": "Deleted successfully",
+    "delete_error": "Failed to delete",
+    "rename_success": "Renamed successfully",
+    "rename_error": "Failed to rename",
+    "download_error": "Failed to download",
+    "not_available": "WebDAV file storage is not available on this server",
+    "cancel": "Cancel",
+    "create": "Create",
+    "save": "Save",
+    "no_results": "No files match your search",
+    "batch_delete_confirm_message": "Are you sure you want to delete {count, plural, one {1 item} other {# items}}? This cannot be undone.",
+    "batch_delete_success": "{count, plural, one {1 item deleted} other {# items deleted}}",
+    "grid_view": "Grid view",
+    "list_view": "List view",
+    "details": "Details",
+    "path": "Path",
+    "preview": "Preview",
+    "preview_error": "Failed to load preview",
+    "cut": "Cut",
+    "copy": "Copy",
+    "paste": "Paste",
+    "move_success": "{count, plural, one {1 item moved} other {# items moved}}",
+    "move_error": "Failed to move",
+    "paste_success": "Pasted successfully",
+    "paste_error": "Failed to paste",
+    "new_text_file": "New Text File",
+    "file_name": "File name",
+    "retry": "Retry",
+    "refresh": "Refresh",
+    "toggle_favorite": "Toggle favorite",
+    "duplicate": "Duplicate",
+    "duplicate_success": "Duplicated successfully",
+    "duplicate_error": "Failed to duplicate",
+    "create_file_success": "File created",
+    "create_file_error": "Failed to create file",
+    "favorites": "Favorites",
+    "recent": "Recent",
+    "properties": "Properties",
+    "open_folder": "Open folder",
+    "upload_folder": "Upload Folder",
+    "file_too_large": "\"{name}\" exceeds the maximum file size ({max})",
+    "undo": "Undo",
+    "undo_success": "Action undone",
+    "undo_error": "Failed to undo",
+    "toolbar": "File actions",
+    "file_list": "Files and folders",
+    "context_menu": "Actions"
   }
 }