feat(playback): restyle the shared player controls as the Hybrid dock

Phase 1 of the "Hybrid" (design variant E) player controls redesign for
app-player-controls, shared by HTML5, Video.js, ArtPlayer and frame-copy
Embedded MPV:

- dock without a surface of its own over a 55% bottom scrim, mirrored by
  the top scrim; fixed --pc-* overlay palette (accent blue, cyan, violet)
- three-column control row with a filled accent play circle; previous /
  -10s / play / +10s / next transport order
- volume slider inline on wide players, behind the popover when compact
  (ControlsLayout, ResizeObserver at 720px of container width)
- drawn timeline track with knob, remaining time (−7:03) instead of the
  total, and a hover time label (ControlsTimelineHover); the range input
  stays as the interaction and accessibility layer
- glass popovers; 40px/12px icon buttons (32px/9px compact)

Docs: player-controls-contract (dock, scrims, layout modes), UI guidelines
palette note. Release note added.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

.changes/playback-hybrid-player-controls.md

@@ -0,0 +1,10 @@
+---
+type: feature
+area: playback
+highlight: Redesigned player controls
+---
+
+The built-in player controls have a new look: a lighter dock that sits
+directly on the video, a round accent play button, the volume slider inline
+on wide players, a remaining-time counter, and a timeline that shows the time
+under your pointer before you seek.