dawcore: <daw-track-controls> themability — CSS vars for hardcoded colors + WCAG AA contrast fix

[naomiaro]

<daw-track-controls> hardcodes rgba whites for the M/S buttons and slider tracks (daw-track-controls.ts:64-145) instead of exposing CSS custom properties, and the nested-opacity slider labels land at 1.97:1 contrast — failing WCAG AA.

• Replace hardcoded rgba values with --daw-* custom properties (button background/border/hover, slider track, thumb shadow)
• Fix slider-label contrast to meet WCAG AA (4.5:1)
• Document the new vars alongside the existing theme list

(Carried over from the dawcore themability-gaps audit note.)