feat(popover): fullscreen-aware portal + migrate the hover surfaces (R8 S4a)

The platform's FloatingPortal targeted <body>, which is painted UNDERNEATH whatever
holds the fullscreen lock. That is why the player's in-bar menus were either
non-portaled (and clipped) or, for add-to-playlist / download, disabled outright in
fullscreen. PopoverSurface now portals into `document.fullscreenElement` while one
exists, via a new reactive `useFullscreenElement()` on the existing WebKit-aware
fullscreen lib.

Two floating-ui details this had to work around: a null `root` means "the container
does not exist yet" (no portal node is created at all), so the no-fullscreen case must
pass undefined; and the portal node is built once and ignores a later root change, so
the portal is keyed on the fullscreen state to remount on a transition.

Migrated the three hand-rolled hover engines onto the platform:
- Tooltip: kept as a passive caption via a new `passthrough` prop on PopoverSurface,
  so it cannot swallow a click aimed at a control it overlaps during the close delay.
  Gains aria-describedby wiring it never had.
- AboutCell: the 160 ms hide-timer hover bridge is now safePolygon; the close-on-page-
  scroll is now hide-and-follow, so the card reappears when the row scrolls back.
- TagManager: same bridge removal; the scroll-fade moves to an inner scroller so the
  mask rides the scrolling box rather than the glass panel.

Verified in-browser: tooltip flip + centering + pointer-transparency, About card flip-
above with the measured height, tag popover right-start anchoring, and the portal
round-trip body -> fullscreen element -> body on a live fullscreenchange.

frontend/src/lib/fullscreen.ts

@@ -1,3 +1,5 @@
+import { useSyncExternalStore } from "react";
+
 // The browser's Fullscreen API, WebKit included.
 //
 // Safari and iPadOS still ship only the prefixed spelling: no `fullscreenchange`, no
@@ -47,3 +49,16 @@ export function onFullscreenChange(handler: () => void): () => void {
     document.removeEventListener("webkitfullscreenchange", handler);
   };
 }
+
+// `fullscreenElement()` returns the live DOM node, so the same element is the same reference across
+// calls — a valid useSyncExternalStore snapshot without any caching of our own.
+const fsSnapshot = () => fullscreenElement() as HTMLElement | null;
+const fsServerSnapshot = () => null;
+
+/** The fullscreen element as reactive state, for code that must RENDER differently while something
+ *  holds the lock — chiefly the popover platform, which has to portal INTO that element (a node
+ *  portaled to <body> is painted underneath the fullscreen element, i.e. invisible). Components that
+ *  merely react to the transition should keep using `onFullscreenChange` directly. */
+export function useFullscreenElement(): HTMLElement | null {
+  return useSyncExternalStore(onFullscreenChange, fsSnapshot, fsServerSnapshot);
+}