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.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

lintunes/preferences.py

@@ -11,6 +11,7 @@ DEFAULTS = {
     "now_playing_font": None,      # None=prefer Century Gothic, ""=use app
                                    # default, "Family"=use app
     "volume": 0.8,                 # logical 0..1 master output volume
+    "visualizer_mode": "on",       # on (color) | dim (gray) | off — click cycle
     # Per-area grayscale overrides (0=black .. 255=white). None = inherit the
     # current/default look for that surface, so nothing changes until the user
     # drags a slider in Preferences.
@@ -19,6 +20,7 @@ DEFAULTS = {
     "color_now_playing_bg": None,    # panel behind the now-playing text
     "color_now_playing_text": None,  # the now-playing title/artist text
     "color_stripe": None,            # alternating row stripes (QPalette.AlternateBase)
+    "color_visualizer_gray": None,   # visualizer bars in dim (gray) mode
     "lastfm": {
         "api_key": "",
         "api_secret": "",