Add server mode awareness to the FreeCAD client. The mode (normal,
read-only, degraded, offline) is fetched from the /ready endpoint on
each status refresh and updated in real-time via SSE server.state events.
Changes:
- Add _server_mode global and _fetch_server_mode() helper that queries
/ready and maps response status to mode string
- Add server_mode_changed signal to SiloEventListener, handle
server.state SSE events in _dispatch()
- Add mode banner to SiloAuthDockWidget: colored bar shown when server
is not in normal mode (yellow=read-only, orange=degraded, red=offline)
- Update _refresh_status() to fetch mode and update banner
- Disable write commands (New, Save, Commit, Push) when server is not
in normal mode via IsActive() checks
Closes#4
