The toolbar download button now uses the already-fetched download URL
from useQuery (zero extra requests), while tree node downloads keep
using useMutation with React Query-managed isPending state instead of
a hand-rolled useState wrapper.
Implement ReadOnlyFilePreview to render sandbox files by type
(code, markdown, image, video, SQLite, unsupported) using existing
skill viewer components with readOnly support. Add
useSandboxFileDownloadUrl and useFetchTextContent hooks for data
fetching, and generalize useFileTypeInfo to accept any file-like
object.
Enhance getFileIconType to accept an extension parameter and cover all
13 FileAppearanceTypeEnum types using an O(1) Map lookup. Update all
call sites to pass the API-provided extension for accurate icon display.
