test: 100% coverage on core modules with headless CI support

- Fix dead exception handler in discord_rpc.py (unreachable DiscordNotFound)
- Replace QMediaPlayer=None stub with proper _StubQMediaPlayer in player.py
- Make AudioVisualizer color stops Qt-free (raw tuples instead of QColor)
- Add conftest QApplication creation check for headless CI detection
- Add test_gui_widgets.py _qt_available() guard for headless environments
- Add test_main.py (30 tests) for main.py entry point and env vars
- Add test_import_fallbacks.py (14 tests) for Qt import fallback paths
- Extend test_gui_helpers.py with _css, _get_thumbnail_nam, _cached_thumb_path tests
- Extend test_discord_rpc.py with direct _connect() test for DiscordNotFound
- Extend test_player_helpers.py with SearchWorker test-mode tests
- Ensure import fallback tests clean up sys.modules to avoid cross-test pollution

100% coverage achieved on: config.py, discord_rpc.py, main.py, music_db.py

tests/test_discord_rpc.py

@@ -257,6 +257,19 @@ class TestDiscordRPC:
             assert result is False
             assert drpc._connected is False
 
+    def test_connect_discord_not_found_direct(self):
+        """_connect handles DiscordNotFound when called directly (not through thread)."""
+        with mock.patch("discord_rpc.Presence") as mock_presence_cls:
+            from discord_rpc import DiscordRPC, DiscordNotFound
+            mock_instance = mock.MagicMock()
+            mock_instance.connect.side_effect = DiscordNotFound()
+            mock_presence_cls.return_value = mock_instance
+
+            drpc = DiscordRPC(client_id="test")
+            result = drpc._connect()
+            assert result is False
+            assert drpc._connected is False
+
     def test_run_reconnects_on_failure(self):
         """_run loop retries connection when _connect fails (covers lines 174-175)."""
         with mock.patch("discord_rpc.Presence") as mock_presence_cls: