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_helpers.py

@@ -158,3 +158,68 @@ class TestArtistsFromJson:
         fn = self._import()
         j = json.dumps([{"name": "Real"}, 42, "str", {"name": "Also Real"}])
         assert fn(j) == "Real, Also Real"
+
+
+# ── _css ───────────────────────────────────────────────────────────────────
+
+class TestCss:
+    """Tests for the _css helper."""
+
+    def _import(self):
+        from gui import _css
+        return _css
+
+    def test_joins_multiple_parts(self):
+        fn = self._import()
+        assert fn("color: red;", "background: blue;") == "color: red;background: blue;"
+
+    def test_single_part(self):
+        fn = self._import()
+        assert fn("color: red;") == "color: red;"
+
+    def test_empty_parts(self):
+        fn = self._import()
+        assert fn() == ""
+
+
+# ── _get_thumbnail_nam ────────────────────────────────────────────────────
+
+class TestGetThumbnailNam:
+    """Tests for the _get_thumbnail_nam helper."""
+
+    def _import(self):
+        from gui import _get_thumbnail_nam
+        return _get_thumbnail_nam
+
+    def test_returns_network_access_manager(self):
+        fn = self._import()
+        nam = fn()
+        # Should return some kind of object (QNetworkAccessManager or MagicMock)
+        assert nam is not None
+
+    def test_is_singleton(self):
+        fn = self._import()
+        nam1 = fn()
+        nam2 = fn()
+        assert nam1 is nam2
+
+
+# ── _cached_thumb_path ────────────────────────────────────────────────────
+
+class TestCachedThumbPath:
+    """Tests for the _cached_thumb_path helper."""
+
+    def _import(self):
+        from gui import _cached_thumb_path, THUMB_CACHE_DIR
+        return _cached_thumb_path, THUMB_CACHE_DIR
+
+    def test_uses_cache_dir(self):
+        fn, cache_dir = self._import()
+        path = fn("video123")
+        assert str(path) == str(cache_dir / "video123.jpg")
+
+    def test_ends_with_jpg(self):
+        fn, _ = self._import()
+        path = fn("abc_def")
+        assert path.suffix == ".jpg"
+        assert "abc_def" in path.name