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/plexPlayerHooks.controls.dom.test.tsx

@@ -0,0 +1,47 @@
+// @vitest-environment jsdom
+//
+// The control bar must not hide while the pointer rests on it. It used to: the 3 s timer only knew
+// "paused" and "a menu is open", so with the mouse parked on the row (about to press Next) the bar
+// faded out from under it (E28 audit F22). YouTube and Plex Web both hold the bar while hovered.
+import { describe, it, expect, vi, afterEach, beforeEach } from "vitest";
+import { act, renderHook } from "@testing-library/react";
+import { useControlsVisibility } from "./plexPlayerHooks";
+
+const setup = () => {
+  const video = { paused: false } as HTMLVideoElement;
+  return renderHook(() =>
+    useControlsVisibility({
+      barAutoHideRef: { current: true },
+      menuOpenRef: { current: false },
+      videoRef: { current: video },
+    }),
+  );
+};
+
+describe("useControlsVisibility", () => {
+  beforeEach(() => vi.useFakeTimers());
+  afterEach(() => vi.useRealTimers());
+
+  it("hides 3 s after the last activity while playing", () => {
+    const { result } = setup();
+    act(() => result.current.wake());
+    act(() => vi.advanceTimersByTime(3000));
+    expect(result.current.uiVisible).toBe(false);
+  });
+
+  it("stays up while the pointer is over the bar, and re-arms the timer when it leaves", () => {
+    const { result } = setup();
+    act(() => {
+      result.current.wake();
+      result.current.holdOnHover(true);
+    });
+    act(() => vi.advanceTimersByTime(10_000));
+    expect(result.current.uiVisible).toBe(true);
+
+    act(() => result.current.holdOnHover(false));
+    act(() => vi.advanceTimersByTime(2900));
+    expect(result.current.uiVisible).toBe(true); // a fresh 3 s from the leave, not from the wake
+    act(() => vi.advanceTimersByTime(100));
+    expect(result.current.uiVisible).toBe(false);
+  });
+});