fix(device-sync): auto-import manifest on mount, clear view on disconnect

- Read psysonic-sync.json automatically when DeviceSync page opens and
  drive is already connected (no manual folder re-select needed)
- Always replace sources from manifest when choosing a folder, so
  switching between sticks loads the correct album list
- Hide source list and status badges when drive is disconnected
- Reset import flag on disconnect so re-plugging triggers a fresh import
- Fix unused `mut` warning in cancel_device_sync

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -1896,7 +1896,7 @@ async fn calculate_sync_payload(
 /// Signals a running `sync_batch_to_device` job to stop after its current tracks finish.
 #[tauri::command]
 fn cancel_device_sync(job_id: String, app: tauri::AppHandle) {
-    if let Ok(mut flags) = sync_cancel_flags().lock() {
+    if let Ok(flags) = sync_cancel_flags().lock() {
         if let Some(flag) = flags.get(&job_id) {
             flag.store(true, Ordering::Relaxed);
         }