Convert cabinet to URL-based routing with extracted UI components
Replace state-driven panel navigation with TanStack Router file-based
routes. Directory rkeys become URL path segments so browsing is
shareable and back-button-friendly (/cabinet/files/abc/def/ghi).
- Add splat route for nested directory navigation
- Extract PanelShell, Breadcrumbs, SegmentedToggle, DropdownMenu
- Convert Sidebar and TopBar to use <Link> instead of callbacks
- Add documents store with WASM directory tree + lazy decryption
- Add WASM buildDirectoryTree with async document name resolution
- Rename kebab-case files to camelCase/PascalCase for consistency