FUSE toggle mid-session is a no-op (mount stays alive)

[ac999]

Toggling FUSE off in the desktop UI calls KD_SetFUSEMode(0), which sets a
flag but takes no action. The FUSE mount stays alive because cgofuse blocks
the goroutine and only allows one mount per process.

Three approaches were investigated:

1. Disconnect+reconnect cycle (interrupts transfers, mount dir persists)
2. Force unmount via fusermount3 (Linux-only, can't remount until restart)
3. Switch notification code path without unmounting (fragile, race potential)

The primary symptom (empty file list when FUSE toggled off) is now fixed:
ADD_FILE and EDIT_FILE both write SyncTracker regardless of FUSE mode.
This issue is about UI/backend state consistency only.