File drops from the file manager insert at the drop position

The table already computed and emitted the drop row for external URL
drops, but PlaylistView._on_files_dropped threw it away, so imported
files always appended. Thread it through: playlist_view.files_dropped
gains a row slot (None = append), MainWindow.import_files and
file_importer.import_paths take an optional position and hand it to
add_tracks_to_playlist, which already supported positional insert.
Files already in the library dedup to the existing track and insert at
the drop position without duplicating the library entry.

tests/test_round18.py

@@ -99,3 +99,75 @@ class TestVisualizerGrayColor:
     def test_visualizer_row_in_preferences_dialog(self, qapp):
         from lintunes.gui.preferences_dialog import _COLOR_ROWS
         assert "color_visualizer_gray" in [key for key, _label in _COLOR_ROWS]
+
+
+# --------------------------------------------------------------------------
+# 2. External file drops insert at the drop position
+# --------------------------------------------------------------------------
+
+def _seeded_manager(tmp_path, track_ids=(1, 2, 3)):
+    from lintunes.models import Library, Track
+    from lintunes.library_manager import LibraryManager
+    tracks = {tid: Track(track_id=tid, name=f"T{tid}") for tid in track_ids}
+    return LibraryManager(Library(tracks=tracks), tmp_path / "data")
+
+
+class TestFileDropPosition:
+    def test_import_paths_inserts_at_position(self, qapp, tmp_path, mp3_file):
+        from lintunes.importers import file_importer
+        manager = _seeded_manager(tmp_path)
+        playlist = manager.create_playlist("Drops")
+        manager.add_tracks_to_playlist(playlist.persistent_id, [1, 2, 3])
+
+        imported = file_importer.import_paths(
+            [mp3_file], tmp_path / "music", manager,
+            playlist.persistent_id, position=1)
+
+        assert len(imported) == 1
+        new_id = imported[0].track_id
+        assert playlist.track_ids == [1, new_id, 2, 3]
+
+    def test_import_paths_default_appends(self, qapp, tmp_path, mp3_file):
+        from lintunes.importers import file_importer
+        manager = _seeded_manager(tmp_path)
+        playlist = manager.create_playlist("Drops")
+        manager.add_tracks_to_playlist(playlist.persistent_id, [1, 2])
+
+        imported = file_importer.import_paths(
+            [mp3_file], tmp_path / "music", manager, playlist.persistent_id)
+
+        assert playlist.track_ids == [1, 2, imported[0].track_id]
+
+    def test_duplicate_file_inserts_without_new_track(self, qapp, tmp_path,
+                                                      mp3_file):
+        from lintunes.importers import file_importer
+        manager = _seeded_manager(tmp_path)
+        existing = file_importer.import_file(
+            mp3_file, tmp_path / "music", manager)
+        playlist = manager.create_playlist("Drops")
+        manager.add_tracks_to_playlist(playlist.persistent_id, [1, 2])
+        count_before = len(manager.library.tracks)
+
+        imported = file_importer.import_paths(
+            [mp3_file], tmp_path / "music", manager,
+            playlist.persistent_id, position=0)
+
+        assert imported[0].track_id == existing.track_id
+        assert playlist.track_ids == [existing.track_id, 1, 2]
+        assert len(manager.library.tracks) == count_before
+
+    def test_playlist_view_relays_drop_row(self, qapp, tmp_path):
+        from lintunes.gui.playlist_view import PlaylistView
+        manager = _seeded_manager(tmp_path)
+        playlist = manager.create_playlist("Drops")
+        view = PlaylistView(manager)
+        view.show_playlist(playlist)
+        received = []
+        view.files_dropped.connect(
+            lambda paths, pid, row: received.append((paths, pid, row)))
+
+        view._on_files_dropped(["/x.mp3"], 2)
+        view._on_files_dropped(["/y.mp3"], None)
+
+        assert received == [(["/x.mp3"], playlist.persistent_id, 2),
+                            (["/y.mp3"], playlist.persistent_id, None)]