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/device_monitor.py

@@ -1,13 +1,25 @@
 import logging
 from typing import Set
 
-from PyQt6.QtCore import QObject, QTimer, pyqtSignal
+from PyQt6.QtCore import QObject, QTimer, QThread, pyqtSignal
 
 from ipod_device import IPodDevice
 
 logger = logging.getLogger(__name__)
 
 
+class _DetectWorker(QThread):
+    finished = pyqtSignal(object)
+
+    def run(self):
+        try:
+            devices = IPodDevice().detect_devices()
+            self.finished.emit(devices)
+        except Exception:
+            logger.exception("Device detection failed")
+            self.finished.emit([])
+
+
 class DeviceMonitor(QObject):
 
     state_changed = pyqtSignal(list)
@@ -17,6 +29,7 @@ class DeviceMonitor(QObject):
         self.timer = QTimer(self)
         self.timer.timeout.connect(self._poll)
         self._device_ids: Set[str] = set()
+        self._poll_worker = None
 
     def start(self, interval_ms: int = 3000):
         self._device_ids.clear()
@@ -25,14 +38,20 @@ class DeviceMonitor(QObject):
 
     def stop(self):
         self.timer.stop()
+        if self._poll_worker and self._poll_worker.isRunning():
+            self._poll_worker.quit()
+            self._poll_worker.wait(3000)
         self._device_ids.clear()
 
     def _poll(self):
-        try:
-            devices = IPodDevice().detect_devices()
-            current_ids = {d["id"] for d in devices}
-            if current_ids != self._device_ids:
-                self._device_ids = current_ids
-                self.state_changed.emit(devices)
-        except Exception:
-            logger.exception("DeviceMonitor poll failed")
+        if self._poll_worker and self._poll_worker.isRunning():
+            return
+        self._poll_worker = _DetectWorker()
+        self._poll_worker.finished.connect(self._on_devices_detected)
+        self._poll_worker.start()
+
+    def _on_devices_detected(self, devices):
+        current_ids = {d["id"] for d in devices}
+        if current_ids != self._device_ids:
+            self._device_ids = current_ids
+            self.state_changed.emit(devices)