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)

lib/jmap/client.ts

@@ -144,6 +144,14 @@ export class JMAPClient {
     this.authHeader = `Bearer ${token}`;
   }
 
+  getAuthHeader(): string {
+    return this.authHeader;
+  }
+
+  getServerUrl(): string {
+    return this.serverUrl;
+  }
+
   private async authenticatedFetch(url: string, init?: Parameters<typeof fetch>[1]): Promise<Response> {
     const headers = { ...init?.headers as Record<string, string>, 'Authorization': this.authHeader };
     let response: Response;