Fix UI freeze on iPod mount/track-load and add content_hash/sync-metadata

- Move mount_device, get_device_info, and track scanning to background
  WorkerThread (new 'mount_and_load' task type) so the UI stays responsive
- Fix DeviceMonitor auto-poll to run detect_devices() in a background thread
- Refactor _on_device_detection_finished and _on_mount_clicked to use the
  new worker; remove synchronous _set_device_mounted
- Avoid double scan (get_track_count + get_all_tracks) — scan once in worker
- Add content_hash fingerprint to _scan_ipod_files and library_cache for
  track matching
- Add update_track_metadata / _write_databases_from_tracks with artwork
  override support in Nano7Database
- Add 'Sync Metadata' button and context menu action in LibraryTab

src/ui/library_tab.py

@@ -214,6 +214,12 @@ class LibraryTab(QWidget):
         self.library_transfer_btn.clicked.connect(self._on_library_transfer_clicked)
         toolbar.addWidget(self.library_transfer_btn)
 
+        self.library_sync_btn = QPushButton("\uD83D\uDD04 Sync Metadata")
+        self.library_sync_btn.setEnabled(False)
+        self.library_sync_btn.setToolTip("Sync metadata and artwork for all tracks on iPod")
+        self.library_sync_btn.clicked.connect(self._on_sync_metadata_clicked)
+        toolbar.addWidget(self.library_sync_btn)
+
         self.library_remove_btn = QPushButton("\uD83D\uDDD1\uFE0F Remove")
         self.library_remove_btn.setEnabled(False)
         self.library_remove_btn.clicked.connect(self._on_library_remove_selected)
@@ -233,6 +239,7 @@ class LibraryTab(QWidget):
             self.library_add_btn,
             self.library_convert_btn,
             self.library_transfer_btn,
+            self.library_sync_btn,
             self.library_remove_btn,
             self.library_edit_btn,
             refresh_btn,
@@ -717,7 +724,9 @@ class LibraryTab(QWidget):
         has_ready = ready_count > 0
         has_source = source_count > 0
 
-        self.library_transfer_btn.setEnabled(has_ready and self._current_mount_point is not None)
+        has_mount = self._current_mount_point is not None
+        self.library_transfer_btn.setEnabled(has_ready and has_mount)
+        self.library_sync_btn.setEnabled(has_ready and has_mount)
         self.library_convert_btn.setEnabled(has_source)
         self.library_remove_btn.setEnabled(len(all_tracks) > 0)
         self.library_edit_btn.setEnabled(len(all_tracks) > 0)
@@ -984,6 +993,8 @@ class LibraryTab(QWidget):
         play_action.triggered.connect(lambda: self._play_track_from_index(row))
 
         if is_ready and self._current_mount_point is not None:
+            sync_action = menu.addAction("\uD83D\uDD04 Sync Metadata to iPod")
+            sync_action.triggered.connect(self._on_sync_metadata_clicked)
             transfer_action = menu.addAction("\uD83D\uDCE4 Transfer to iPod")
             transfer_action.triggered.connect(self._on_library_transfer_clicked)
         else:
@@ -1129,6 +1140,84 @@ class LibraryTab(QWidget):
 
         self._cleanup_worker()
 
+    def _on_sync_metadata_clicked(self):
+        selected_tracks = self._get_selected_ready_tracks()
+        if not selected_tracks:
+            QMessageBox.warning(self, "Error", "No tracks selected.")
+            return
+
+        if not self._current_mount_point:
+            QMessageBox.warning(self, "Error", "No iPod device mounted.")
+            return
+
+        if self.worker_thread and self.worker_thread.isRunning():
+            QMessageBox.information(self, "Info", "Please wait for the current task to finish")
+            return
+
+        self.library_sync_btn.setEnabled(False)
+        self.library_status.setText("Matching tracks on iPod...")
+        self.library_progress.setValue(0)
+
+        from ipod_nano7_db import Nano7Database, content_hash as compute_content_hash
+
+        db = Nano7Database(self._current_mount_point)
+        ipod_tracks = db.get_all_tracks()
+        ipod_by_hash = {t.get("content_hash", ""): t for t in ipod_tracks if t.get("content_hash")}
+
+        sync_batch = []
+        errors = []
+        for track_info, local_path in selected_tracks:
+            ch = compute_content_hash(local_path)
+            ipod_track = ipod_by_hash.get(ch)
+            if ipod_track is None:
+                ipod_track = db.find_track_by_metadata(
+                    track_info.title, track_info.artist, track_info.album or "",
+                )
+            if ipod_track is None:
+                errors.append(f"'{track_info.title}' — not found on iPod")
+                continue
+
+            sync_batch.append({
+                "ipod_pid": ipod_track["pid"],
+                "local_path": local_path,
+                "new_metadata": {
+                    "title": track_info.title,
+                    "artist": track_info.artist,
+                    "album": track_info.album or "",
+                    "track_number": track_info.track_number or 0,
+                    "genre": track_info.genre or "",
+                },
+            })
+
+        if not sync_batch:
+            msg = "No matching tracks found on iPod"
+            if errors:
+                msg += ":\n" + "\n".join(errors[:5])
+            QMessageBox.warning(self, "Sync Error", msg)
+            return
+
+        if errors:
+            logger.warning("Sync metadata skipped for %d unmatched track(s)", len(errors))
+
+        self.worker_thread = WorkerThread(
+            task_type="sync_metadata",
+            sync_batch=sync_batch,
+            mount_point=self._current_mount_point,
+        )
+        self.worker_thread.progress_signal.connect(self._on_library_progress)
+        self.worker_thread.finished_signal.connect(self._on_sync_metadata_finished)
+        self.worker_thread.start()
+
+    def _on_sync_metadata_finished(self, success, message, result):
+        self.library_sync_btn.setEnabled(True)
+        if success:
+            self.library_status.setText(message)
+            QMessageBox.information(self, "Sync Complete", message)
+        else:
+            self.library_status.setText(f"Error: {message}")
+            QMessageBox.warning(self, "Sync Error", message)
+        self._cleanup_worker()
+
     def _cleanup_worker(self):
         if self.worker_thread:
             self.worker_thread.wait(3000)
@@ -1168,6 +1257,7 @@ class LibraryTab(QWidget):
     def on_device_unmounted(self):
         self._current_mount_point = None
         self.library_transfer_btn.setEnabled(False)
+        self.library_sync_btn.setEnabled(False)
         self.library_progress.setVisible(False)
         self.library_status.setVisible(False)