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_gui_widgets.py

@@ -14,12 +14,26 @@ from unittest import mock
 import pytest
 
 # The conftest stubs PySide6.QtWidgets as a MagicMock when Qt is
-# unavailable (headless CI).  ``importorskip`` succeeds on the mock
-# module, so we check the actual class type instead.
+# unavailable (headless CI).  We check whether we have a real Qt runtime
+# by verifying QApplication is not a MagicMock and that it can be created.
 from PySide6.QtWidgets import QApplication, QWidget
 
-if isinstance(QApplication, mock.MagicMock):
-    pytest.skip("Real Qt runtime not available (stubs active)",
+def _qt_available() -> bool:
+    """Return True if a real QApplication can be created (display available)."""
+    if isinstance(QApplication, mock.MagicMock):
+        return False
+    inst = QApplication.instance()
+    if inst is not None:
+        return True
+    try:
+        app = QApplication([])
+        app.quit()
+        return True
+    except (RuntimeError, OSError):
+        return False
+
+if not _qt_available():
+    pytest.skip("Real Qt runtime not available (stubs active / headless)",
                 allow_module_level=True)
 
 from PySide6.QtCore import QTimer