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/conftest.py

@@ -176,12 +176,25 @@ def _install_stubs() -> None:
 
 _QT_AVAILABLE = False
 try:
-    # Attempt real import — this will fail in headless CI.
+    # Step 1: Try to import PySide6 modules.  These can succeed even in
+    # headless CI because no display is needed to import the Python bindings.
     import PySide6  # noqa: F401
     import PySide6.QtCore  # noqa: F401
     import PySide6.QtMultimedia  # noqa: F401
+
+    # Step 2: Verify that a QApplication can actually be created.
+    # In headless CI without QT_QPA_PLATFORM=offscreen this will raise
+    # a RuntimeError ("Cannot create a QWidget without a QApplication")
+    # or OSError ("Could not connect to display").
+    from PySide6.QtWidgets import QApplication
+    _existing_app = QApplication.instance()
+    if _existing_app is None:
+        _test_app = QApplication([])
+        _test_app.quit()
+        del _test_app
+
     _QT_AVAILABLE = True
-except (ImportError, OSError):
+except (ImportError, OSError, RuntimeError):
     pass
 
 if not _QT_AVAILABLE: