Fix Add Files flow: manual source files now persist across scans

- Add self.library_source_files list to store manually added file paths
- _on_library_add_files now adds to library_source_files instead of
  calling _scan_library() (which would clear the list)
- _refresh_source_ui merges scanned sources + library_source_files
  with dedup by path. All items get proper UserRole data.
- _on_library_remove_selected now removes from library_source_files too
- Previously added files would be lost on every scan because
  _scan_library() called .clear() on the source list

src/main.py

@@ -216,6 +216,7 @@ class MainWindow(QMainWindow):
         self.worker_thread = None
         self.downloaded_tracks: List[TrackInfo] = []
         self.library_ready: List[Dict] = []
+        self.library_source_files: List[str] = []
         self.ipod_devices: List[Dict] = []
         self.current_mount_point: Optional[str] = None
 
@@ -560,15 +561,54 @@ class MainWindow(QMainWindow):
             self.library_status.setText("No tracks ready — convert source files or download")
             self.library_transfer_btn.setEnabled(False)
 
-    def _refresh_source_ui(self, sources):
+    def _refresh_source_ui(self, scanned_sources):
+        """Build the Source Files list by combining scanned files + manual additions.
+        
+        Deduplicates by path. All items get UserRole data for _get_selected_source_files().
+        """
         self.library_source_list.clear()
-        for src in sorted(sources, key=lambda s: (s["artist"], s["title"])):
+        
+        seen_paths = set()
+        all_sources = []
+
+        for src in scanned_sources:
+            path = src.get("path", "")
+            if path and path not in seen_paths and os.path.exists(path):
+                seen_paths.add(path)
+                all_sources.append(src)
+
+        for fpath in self.library_source_files:
+            if fpath in seen_paths:
+                continue
+            if not os.path.exists(fpath):
+                continue
+            fname = os.path.basename(fpath)
+            size = os.path.getsize(fpath)
+            try:
+                handler = MetadataHandler()
+                info = handler.extract_tags(fpath)
+                title = info.title if info else fname
+                artist = info.artist if info else "Unknown"
+            except Exception:
+                title = fname
+                artist = "Unknown"
+
+            src = {
+                "path": fpath,
+                "title": title,
+                "artist": artist,
+                "size": size,
+            }
+            all_sources.append(src)
+            seen_paths.add(fpath)
+
+        for src in sorted(all_sources, key=lambda s: (s["artist"], s["title"])):
             display = f"{src['artist']}  —  {src['title']}     {self._format_size(src['size'])}"
             item = QListWidgetItem(display)
             item.setData(Qt.ItemDataRole.UserRole, src)
             self.library_source_list.addItem(item)
 
-        count = len(sources)
+        count = len(all_sources)
         if count:
             self.library_convert_btn.setEnabled(True)
         else:
@@ -620,15 +660,19 @@ class MainWindow(QMainWindow):
         self._scan_library()
 
     def _on_library_add_files(self):
-        audio_filter = "Audio Files (*.flac *.wav *.ogg *.opus *.wma *.ape *.aiff *.alac)"
+        source_filter = "Audio Files (*.flac *.wav *.ogg *.opus *.wma *.ape *.aiff *.alac)"
         files, _ = QFileDialog.getOpenFileNames(
-            self, "Select Audio Files", "", audio_filter
+            self, "Select Audio Files", "", source_filter
         )
+        new_files = []
         for f in files:
-            existing = self.library_source_list.findItems(f, Qt.MatchFlag.MatchExactly)
+            ext = os.path.splitext(f)[1].lower()
-            if not existing:
+            if ext in SOURCE_EXTS and os.path.exists(f) and f not in self.library_source_files:
-                self.library_source_list.addItem(f)
+                self.library_source_files.append(f)
-        self._scan_library()
+                new_files.append(f)
+
+        if new_files:
+            self._refresh_source_ui([])
 
     def _on_library_remove_selected(self):
         """Delete selected files from disk and clean up empty directories."""
@@ -666,6 +710,8 @@ class MainWindow(QMainWindow):
             try:
                 os.remove(path)
                 deleted += 1
+                if path in self.library_source_files:
+                    self.library_source_files.remove(path)
                 parent = os.path.dirname(path)
                 while parent != os.path.dirname(parent):
                     if os.listdir(parent):