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)
