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)
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