fix(player): the Plex bars hold while hovered, volume is the native slider, tooltips no longer re-layer the row (E28)

- F22: the control bar faded out from under a mouse parked on it; the hide timer
  now waits while the pointer is on either band and re-arms on leave (test seen
  RED by disabling the hold).
- The hand-drawn volume bar broke DESIGN.md A6 (native sliders): it had no hover,
  focus, keyboard or screen-reader state. It is the Slider primitive now; a
  pointer drag blurs it so Space and the arrows keep driving the player.
- The control tooltips are composited from mount (will-change: opacity): a fade
  that promoted one on hover re-layered its neighbours under the player's
  scale(1.25), and the whole row shimmered.

frontend/src/lib/plexPlayerPrefs.ts

@@ -74,7 +74,6 @@ export const DEFAULT_PREFS: PlexPlayerPrefs = {
   autoSkipDelay: 5,
   upNext: true,
 };
-export const clamp01 = (n: number) => Math.max(0, Math.min(1, n));
 
 // A punchy subtitle shadow. The old single 0-offset blur washed out to nothing on bright scenes even
 // at the max slider value; instead lay down eight offset copies (a solid dark OUTLINE around the