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

gui.py

@@ -607,16 +607,19 @@ class AudioVisualizer(QWidget):
     _TICK_MS = 16
 
     # Spectral gradient colour stops  (left → right / bass → treble)
-    SPECTRUM_COLORS = [
-        QColor(139, 92, 246),   # 0.0  – Violet  (bass)
-        QColor(168, 85, 247),   # 0.1  – Purple
-        QColor(236, 72, 153),   # 0.25 – Pink
-        QColor(251, 146, 134),  # 0.4  – Rose
-        QColor(251, 207, 132),  # 0.5  – Amber   (centre)
-        QColor(251, 207, 132),  # 0.6  – Amber
-        QColor(236, 72, 153),   # 0.75 – Pink
-        QColor(168, 85, 247),   # 0.9  – Purple
-        QColor(139, 92, 246),   # 1.0  – Violet  (bass mirror)
+    # Stored as raw (R, G, B) tuples to avoid Qt dependency at class-definition
+    # time — needed so the module can be imported for pure-helper-function tests
+    # without a real PySide6 runtime.
+    _SPECTRUM_COLOR_STOPS = [
+        (139, 92, 246),   # 0.0  – Violet  (bass)
+        (168, 85, 247),   # 0.1  – Purple
+        (236, 72, 153),   # 0.25 – Pink
+        (251, 146, 134),  # 0.4  – Rose
+        (251, 207, 132),  # 0.5  – Amber   (centre)
+        (251, 207, 132),  # 0.6  – Amber
+        (236, 72, 153),   # 0.75 – Pink
+        (168, 85, 247),   # 0.9  – Purple
+        (139, 92, 246),   # 1.0  – Violet  (bass mirror)
     ]
 
     def __init__(self, parent=None):
@@ -887,21 +890,23 @@ class AudioVisualizer(QWidget):
 
     @staticmethod
     def _spectrum_color(t: float) -> QColor:
-        cols = AudioVisualizer.SPECTRUM_COLORS
+        """Interpolate between colour stops and return a ``QColor``."""
+        stops = AudioVisualizer._SPECTRUM_COLOR_STOPS
         if t <= 0.0:
-            return cols[0]
+            r, g, b = stops[0]
+            return QColor(r, g, b)
         if t >= 1.0:
-            return cols[-1]
-        t_scaled = t * (len(cols) - 1)
+            r, g, b = stops[-1]
+            return QColor(r, g, b)
+        t_scaled = t * (len(stops) - 1)
         idx = int(t_scaled)
         frac = t_scaled - idx
-        c1 = cols[idx]
-        c2 = cols[min(idx + 1, len(cols) - 1)]
+        c1 = stops[idx]
+        c2 = stops[min(idx + 1, len(stops) - 1)]
         return QColor(
-            int(c1.red()   + (c2.red()   - c1.red())   * frac),
-            int(c1.green() + (c2.green() - c1.green()) * frac),
-            int(c1.blue()  + (c2.blue()  - c1.blue())  * frac),
-            int(c1.alpha() + (c2.alpha() - c1.alpha()) * frac),
+            int(c1[0] + (c2[0] - c1[0]) * frac),
+            int(c1[1] + (c2[1] - c1[1]) * frac),
+            int(c1[2] + (c2[2] - c1[2]) * frac),
         )