Add split-panel file preview, UI cleanup, and fix document name flicker [CL-204] [CL-206] [CL-264] [CL-286]
Split-panel preview: PanelShell renders two separate panels via sidePanel
prop — side-by-side on large screens, stacked on mobile. Active file
highlighted in list/grid. PreviewPaneHeader shows document name, download,
and close controls.
Image preview: YARL lightbox with Phosphor zoom icons, top-aligned,
OpakeLogoSquares loading skeleton, client-side image resizing.
Markdown preview: react-markdown with remark-gfm, syntax highlighting via
rehype-highlight, prose styling.
Directory README: GitHub-style readme.md rendering above file list with
expand/collapse at 300px.
Flicker fix: fetchAll swaps items atomically instead of clearing to empty
first, preventing skeleton flash on refetch.
UI cleanup: remove E2E encrypted badge, storage indicator, Trash and
Encrypted sidebar items. Notification bell opens empty portal dropdown
without unread indicator. User menu uses KeyIcon linking to /devices/.
DropdownMenu converted to portal-based rendering.