Add volume slider to the transport bar

A slim horizontal master-volume slider in its own slot between the
visualizer and the timeline (TASKS.md). Perceptual->linear gain so it
ramps like iTunes; persists to preferences.json (debounced so it doesn't
thrash the theme). Clicking jumps to the spot via a new ClickJumpSlider
base extracted from SeekSlider.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lintunes/preferences.py

@@ -10,6 +10,7 @@ DEFAULTS = {
     "highlight": "blue",           # see theme.HIGHLIGHT_COLORS
     "now_playing_font": None,      # None=prefer Century Gothic, ""=use app
                                    # default, "Family"=user-chosen substitute
+    "volume": 0.8,                 # logical 0..1 master output volume
     "lastfm": {
         "api_key": "",
         "api_secret": "",