Visualizer: gray-mode color slider + mode persists across launches

- New prefs: visualizer_mode (on/dim/off, saved on every click) and
  color_visualizer_gray (None = keep the derived alternateBase look).
- VisualizerWidget takes prefs (optional, so existing call sites/tests
  keep working); restores its mode at startup and validates junk values.
- New "Visualizer (gray mode):" row in the Preferences color sliders;
  theme.default_gray mirrors the widget's 0.88-lightness derivation so
  an untouched slider shows the current appearance.
- TransportBar.refresh_theme() repaints the visualizer so slider drags
  preview live.

lintunes/theme.py

@@ -70,7 +70,17 @@ def default_gray(key: str) -> int:
     current appearance, so showing the slider doesn't imply a change."""
     if key in _FIXED_GRAY_DEFAULT:
         return _FIXED_GRAY_DEFAULT[key]
-    color = _base().color(QPalette.ColorGroup.Active, _PALETTE_GRAY_ROLE[key])
+    if key == "color_visualizer_gray":
+        # Dim-mode bars derive from alternateBase dropped ~12% in lightness
+        # (see VisualizerWidget._bar_color); mirror that so the slider shows
+        # the current appearance.
+        color = _base().color(QPalette.ColorGroup.Active,
+                              QPalette.ColorRole.AlternateBase)
+        h, s, l, a = color.getHslF()
+        color.setHslF(h, s, l * 0.88, a)
+    else:
+        color = _base().color(QPalette.ColorGroup.Active,
+                              _PALETTE_GRAY_ROLE[key])
     return round(0.299 * color.red() + 0.587 * color.green()
                  + 0.114 * color.blue())