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/components/plexPlayerControls.tsx

@@ -133,6 +133,10 @@ export function Ctrl({
 }) {
   const pos =
     align === "start" ? "left-0" : align === "end" ? "right-0" : "left-1/2 -translate-x-1/2";
+  // `will-change-[opacity]` keeps each tooltip on its own compositor layer from the start. Without it
+  // the browser promotes a tooltip only while its fade runs, and inside the player's scale(1.25)
+  // the neighbouring glyphs got re-layered with it: every button in the row shimmered as the
+  // pointer crossed one (owner, E28 audit).
   return (
     <div className="relative flex group/ctrl">
       <IconButton
@@ -147,7 +151,7 @@ export function Ctrl({
         {children}
       </IconButton>
       <span
-        className={`pointer-events-none absolute bottom-full ${pos} mb-2 whitespace-nowrap rounded-sm bg-black/90 px-2 py-1 text-2xs text-white opacity-0 transition group-hover/ctrl:opacity-100`}
+        className={`pointer-events-none absolute bottom-full ${pos} mb-2 whitespace-nowrap rounded-sm bg-black/90 px-2 py-1 text-2xs text-white opacity-0 will-change-[opacity] transition group-hover/ctrl:opacity-100`}
       >
         {label}
       </span>