Harden Library tab thread cleanup (covers + items)

Library covers used the same QThread+worker architecture as the store but
never deleted finished threads/loaders, so every reload leaked one QThread
and one CoverLoader. Library item loading leaked its QThread too.

CoverShowcase now removes + deleteLater's loaders on their finished signal
and deletes their threads on thread.finished; MainWindow.load_library deletes
the finished worker and thread instead of only untracking them.

app/gui.py

@@ -463,7 +463,9 @@ class CoverShowcase(QWidget):
         self.loader.loaded.connect(self._on_loaded)
         self.loader.failed.connect(self._on_failed)
         self.loader.finished.connect(self._on_loading_done)
+        self.loader.finished.connect(self._on_loader_finished)
         self.loader.finished.connect(self.thread.quit)
+        self.thread.finished.connect(self._on_thread_finished)
         self._threads.append(self.thread)
         self._loaders.append(self.loader)
         self.thread.start()
@@ -521,6 +523,26 @@ class CoverShowcase(QWidget):
         self._loading = False
         self._update_status()
 
+    def _on_loader_finished(self):
+        """Drop a finished cover loader from tracking and schedule its deletion."""
+        loader = self.sender()
+        if loader is None:
+            return
+        if loader in self._loaders:
+            self._loaders.remove(loader)
+        if self.loader is loader:
+            self.loader = None
+        loader.deleteLater()
+
+    def _on_thread_finished(self):
+        """Delete a finished cover-loader thread."""
+        thread = self.sender()
+        if thread in self._threads:
+            self._threads.remove(thread)
+        if self.thread is thread:
+            self.thread = None
+        thread.deleteLater()
+
     def _update_status(self):
         if self._closing:
             return
@@ -4790,6 +4812,8 @@ class MainWindow(QMainWindow):
         thread.started.connect(worker.run)
         worker.result.connect(self._on_library_loaded)
         worker.finished.connect(thread.quit)
+        worker.finished.connect(worker.deleteLater)
+        thread.finished.connect(thread.deleteLater)
         thread.finished.connect(lambda t=thread, w=worker: self._forget_library_thread(t, w))
         self._library_threads.append(thread)
         self._library_workers.append(worker)