feat(media): add toggle for album art accent colors (#2831) (#2832)

* fix(media): use system colours in player instead of album cover accent

Closes #2831

Removes ColorQuantizer-based album art accent extraction from
MediaAccentService. All accent properties now return Theme.primary
and Theme.onPrimary directly, so the Dank Dash player always
matches the system colour scheme.

Blame: ee6f7b47 (introduced MediaAccentService & ColorQuantizer)
       d799175c (tweaked seekbar accent colours)
       a62ae336 (further integrated accent into player/album art)
       c44ffae7 (monochrome art edge case fixes)

* feat(media): add toggle for album art accent colours (#2831)

Adds mediaUseAlbumArtAccent setting (default: off) to Settings.
When enabled, MediaAccentService extracts accent from album art
via ColorQuantizer. When disabled, uses Theme.primary.
Toggle is in Settings > Media Player.

* fix: use american english spelling (colour -> color)

Port 1.5

(cherry picked from commit e2b3a2e3ca)

quickshell/Common/settings/SettingsSpec.js

@@ -185,6 +185,7 @@ var SPEC = {
     scrollTitleEnabled: { def: true },
     mediaAdaptiveWidthEnabled: { def: true },
     audioVisualizerEnabled: { def: true },
+    mediaUseAlbumArtAccent: { def: false },
     audioScrollMode: { def: "volume" },
     audioWheelScrollAmount: { def: 5 },
     audioDeviceScrollVolumeEnabled: { def: false },