fix(plex): the player's stage follows the tablet toolbar, and its bands stay out of a double-tap

- The stage is sized in dvh (80dvh × 1.25 windowed, 100dvh fullscreen); the viewport-units
  contract no longer exempts the player.
- A tap that reveals the bands keeps them pointer-events-none for the double-tap window, so the
  second tap of a skip at the bottom is not a seek.
- An arrow on the volume slider wakes the bands, and a keyboard focus in a band holds them
  (`holdOnFocus`, `:focus-visible` only, so a clicked button does not pin the bar).
- Next stays mounted (disabled) while the next episode's detail loads, keeping the focus.

frontend/src/lib/plexPlayerHooks.ts

@@ -72,7 +72,8 @@ export function useFullscreen(wrapRef: RefObject<HTMLElement | null>): {
 // bar up so it can't vanish mid-choice), and the <video> paused state (a paused video keeps the bar).
 // `holdOnHover` is the pointer resting on the chrome: the bar stays while it is there (as on YouTube
 // and Plex Web — E28 F22, it used to fade out from under a mouse parked on Next), and leaving it
-// starts a fresh 3 s.
+// starts a fresh 3 s. `holdOnFocus` is the same for a KEYBOARD focus in the chrome (E28 review E6): the
+// bar must not fade from under the control a key user is on.
 //
 // On touch (C3) nothing moves a mouse, so the timer is also armed by PLAYBACK starting — before, the
 // bar stayed up until the first tap however long the video played — and a tap on the picture only
@@ -86,6 +87,7 @@ export function useControlsVisibility(opts: {
   uiVisible: boolean;
   wake: () => void;
   holdOnHover: (over: boolean) => void;
+  holdOnFocus: (on: boolean) => void;
   toggle: () => void;
   show: (visible: boolean) => void;
 } {
@@ -94,6 +96,7 @@ export function useControlsVisibility(opts: {
   const visibleRef = useRef(true);
   const hideTimer = useRef<number | null>(null);
   const hovered = useRef(false);
+  const focused = useRef(false);
   const setVisible = useCallback((v: boolean) => {
     visibleRef.current = v;
     setUiVisible(v);
@@ -104,7 +107,8 @@ export function useControlsVisibility(opts: {
     if (!barAutoHideRef.current) return; // per-user setting: never auto-hide the control bar
     hideTimer.current = window.setTimeout(() => {
       // Keep the controls up while the audio/subtitle menu is open, else it would vanish mid-choice.
-      if (!videoRef.current?.paused && !menuOpenRef.current && !hovered.current) setVisible(false);
+      if (!videoRef.current?.paused && !menuOpenRef.current && !hovered.current && !focused.current)
+        setVisible(false);
     }, 3000);
   }, [barAutoHideRef, menuOpenRef, videoRef, setVisible]);
   const show = useCallback(
@@ -124,6 +128,13 @@ export function useControlsVisibility(opts: {
     },
     [wake],
   );
+  const holdOnFocus = useCallback(
+    (on: boolean) => {
+      focused.current = on;
+      if (!on) wake();
+    },
+    [wake],
+  );
   useEffect(() => {
     const video = videoRef.current;
     if (!video) return;
@@ -136,7 +147,7 @@ export function useControlsVisibility(opts: {
     },
     [],
   );
-  return { uiVisible, wake, holdOnHover, toggle, show };
+  return { uiVisible, wake, holdOnHover, holdOnFocus, toggle, show };
 }
 
 // Periodic + on-unmount progress checkpoint. Reads live position/duration through refs (owned by the