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

player.py

@@ -58,13 +58,62 @@ try:
 except (ImportError, OSError):
     _HAS_QT = False
     QObject = object
-    QThread = type("QThread", (), {"start": lambda s: None,
-                                   "quit": lambda s: None,
-                                   "wait": lambda s, t=3000: True})
+
+    class _StubQThread:
+        """Stub replacement for PySide6.QtCore.QThread."""
+        def start(self) -> None: pass
+        def quit(self) -> None: pass
+        def wait(self, timeout: int = 3000) -> bool: return True
+    QThread = _StubQThread
+
     Signal = lambda *a: lambda f: f
     Slot = lambda *a: lambda f: f
     QUrl = None
-    QMediaPlayer = None
+
+    class _StubPlaybackState:
+        StoppedState = 0
+        PlayingState = 1
+        PausedState = 2
+
+    class _StubMediaStatus:
+        NoMedia = 0
+        LoadingMedia = 1
+        LoadedMedia = 2
+        StalledMedia = 3
+        BufferingMedia = 4
+        BufferedMedia = 5
+        EndOfMedia = 6
+        InvalidMedia = 7
+
+    class _StubError:
+        NoError = 0
+        ResourceError = 1
+        FormatError = 2
+        NetworkError = 3
+        AccessDeniedError = 4
+        ServiceMissingError = 5
+
+    class _StubQMediaPlayer:
+        PlaybackState = _StubPlaybackState
+        MediaStatus = _StubMediaStatus
+        Error = _StubError
+
+        def __init__(self):
+            self._playback_state = _StubPlaybackState.StoppedState
+
+        def setSource(self, url) -> None: pass
+        def play(self) -> None: pass
+        def pause(self) -> None: pass
+        def stop(self) -> None: pass
+        def setPosition(self, ms: int) -> None: pass
+        def position(self) -> int: return 0
+        def duration(self) -> int: return 0
+        def playbackState(self) -> int:
+            return self._playback_state
+        def audioOutput(self): return None
+        def setAudioOutput(self, output) -> None: pass
+
+    QMediaPlayer = _StubQMediaPlayer
     QAudioOutput = None
 
 log = logging.getLogger("tunetti.player")