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/components/AnchoredPopover.tsx

@@ -35,6 +35,7 @@ import {
   type UseRoleProps,
 } from "@floating-ui/react";
 import { useLayer } from "../lib/layerStack";
+import { useFullscreenElement } from "../lib/fullscreen";
 
 // R8 popover platform. ONE anchored-popover primitive over @floating-ui/react, replacing the ~9
 // hand-rolled positioning engines (measured flip / width clamp / rAF scroll-follow / scroller-relative
@@ -271,6 +272,7 @@ export function PopoverSurface({
   children,
   focusManaged,
   initialFocus,
+  passthrough,
 }: {
   popover: AnchoredPopover;
   className?: string;
@@ -278,6 +280,10 @@ export function PopoverSurface({
   children: ReactNode;
   focusManaged?: boolean;
   initialFocus?: number | React.MutableRefObject<HTMLElement | null>;
+  /** The surface never receives pointer events — for a passive caption that must not swallow a click
+   *  aimed at whatever it happens to cover during its close delay. Interactive hover cards (scrollable
+   *  content, links) must NOT set this. */
+  passthrough?: boolean;
 }) {
   const {
     context,
@@ -295,6 +301,11 @@ export function PopoverSurface({
     elementsRef,
     labelsRef,
   } = popover;
+  // A node portaled to <body> is painted UNDERNEATH whatever holds the fullscreen lock, so a popover
+  // opened from a player's control bar in fullscreen would be invisible. Portal into the fullscreen
+  // element instead while one exists — this is what let the player's in-bar menus stop being either
+  // non-portaled (and clipped) or outright disabled in fullscreen.
+  const fsRoot = useFullscreenElement();
   const { style: floatingPropStyle, ...floatingProps } = getFloatingProps() as {
     style?: CSSProperties;
   } & Record<string, unknown>;
@@ -317,8 +328,9 @@ export function PopoverSurface({
         // Hide until the first position is measured (else it flashes at 0,0 before the transform
         // lands) and while the trigger is scrolled out of its clipping scroller (hide-and-follow).
         ...(!isPositioned || referenceHidden
-          ? { visibility: "hidden", pointerEvents: "none" }
+          ? { visibility: "hidden" as const, pointerEvents: "none" as const }
           : null),
+        ...(passthrough ? { pointerEvents: "none" as const } : null),
       }}
     >
       {/* Inner wrapper carries the visual styling + pop-in animation. Keeping the ANIMATION off the
@@ -345,7 +357,11 @@ export function PopoverSurface({
 
   return (
     <FloatingNode id={nodeId}>
-      <FloatingPortal>
+      {/* `root` must be undefined (NOT null) to mean "default to <body>" — floating-ui reads a null
+          root as "the container doesn't exist yet" and creates no portal node at all. And it only
+          builds that node once, so a root that changes mid-open is ignored; the key remounts the
+          portal on a fullscreen transition, which is exactly when the popover must move. */}
+      <FloatingPortal key={fsRoot ? "fullscreen" : "body"} root={fsRoot ?? undefined}>
         {managed ? (
           <FloatingFocusManager
             context={context}