- Add cista/fileserver.py: REST blueprint at /files with PUT upload,
DELETE, MKCOL, POST cp/mv (combined), GET/HEAD static serving
- Remove WS control handler and all ControlBase/Cmd protocol types
- Frontend: SelectionToolbar, FileExplorer, Gallery now POST to /files
instead of opening a control WebSocket per operation
- Remove controlUrl export from WS.ts
- Add tests: REST API, static streaming, path/escaping security
- Catch ValueError from filename.sanitize and return 400 Bad Request
