Adds a folder picker (webkitdirectory) and drag-and-drop directory
support (File System Entries API), preserving relative paths via
a new optional relative_path field on the upload endpoint.
- client.test.ts: use globalThis.fetch instead of global.fetch and drop the
unused describe import, fixing the broken `npm run build`
- DocumentEditor: separate load-error (replaces view) from save-error (shown
inline, editor and unsaved content stay mounted so a failed save no longer
destroys in-progress edits); expose dirty state via onDirtyChange
- App: prompt via window.confirm before switching documents while dirty;
increment a refreshKey on successful upload so FileExplorer re-fetches the
tree; track isSearching so the "no results" overlay doesn't flash during
the search debounce window
- FileExplorer: accept a refreshKey prop to force a root-listing re-fetch
- SearchBar: convert to a controlled component (value prop) so App can clear
the input after jumping to a search result, fixing input/state desync
- UploadButton: surface upload failures inline instead of swallowing the
rejected promise, and keep the modal open for retry
