feat(overlay): shared layer-stack for topmost-only Escape across modals, popovers, players (R8 S3)

Replace the three mutually-unaware Escape systems — Modal's private modalStack,
the popovers' document listeners with a blanket stopPropagation, and the two
players' modalCount() checks — with one shared lib/layerStack.ts. Every open
dismissable layer registers in open order; a layer acts on Escape only when it
is topmost, and reads anyLayerAbove() to decide whether to defer. Coordination
is now a stack lookup, not event-propagation order, so it no longer depends on
which listener happens to fire first.

- Modal + useDismiss register via the stack; useDismiss drops the blanket
  stopPropagation (its documented side effect: it swallowed a player's own
  Escape while any popover was open).
- Both players register as a layer and defer via anyLayerAbove() instead of
  modalCount(); PlayerModal checks it before its maximise/fullscreen steps.
  A player MENU is a layer too, so it closes first and the next Escape reaches
  the auto-skip-cancel — fixing the documented PlexPlayer bug.
- AnchoredPopover routes non-hover Escape through the stack (floating-ui's own
  escapeKey off there) and registers as a layer, so a player/modal underneath
  defers once S4 migrates the in-player menus. FloatingTree still owns
  popover-vs-popover ordering; the stack adds the cross-type ordering.

frontend/src/components/PlayerModal.tsx

@@ -27,7 +27,7 @@ import {
 } from "lucide-react";
 import Avatar from "./Avatar";
 import AddToPlaylist from "./AddToPlaylist";
-import { modalCount } from "./Modal";
+import { pushLayer, removeLayer, anyLayerAbove } from "../lib/layerStack";
 import DownloadButton from "./DownloadButton";
 import { api, type Video, type VideoStatus } from "../lib/api";
 import type { YTNamespace, YTPlayer, YTPlayerEvent } from "../lib/youtube";
@@ -490,12 +490,28 @@ export default function PlayerModal({
     staleTime: 5 * 60_000,
   });
 
+  // This player is a dismissable layer: register it so a Modal or popover opened ABOVE it (e.g. the
+  // download dialog, or — after S4 — an anchored menu) takes Escape first and this player defers.
+  const layerIdRef = useRef<number | null>(null);
+  useEffect(() => {
+    const id = pushLayer();
+    layerIdRef.current = id;
+    return () => {
+      removeLayer(id);
+      layerIdRef.current = null;
+    };
+  }, []);
+
   // Keyboard shortcuts (Esc close / F fullscreen / Space play-pause) + background scroll lock.
   // These fire only while focus is on our page, not inside the cross-origin player iframe — so
   // we focus the modal card on open and again on player-ready (autoplay can grab focus).
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
       if (e.key === "Escape") {
+        // Defer to any layer opened above the player FIRST (a Modal like the download dialog, or an
+        // anchored popover): it takes the Escape and we do nothing — so we don't also step out of
+        // maximise or close the player underneath it.
+        if (layerIdRef.current != null && anyLayerAbove(layerIdRef.current)) return;
         // In fullscreen, let the browser's Esc exit fullscreen only — don't also close the modal.
         if (fullscreenElement()) return;
         // Same for our own maximise: Esc steps back out of it before it closes the player.
@@ -504,9 +520,6 @@ export default function PlayerModal({
           setMaximized(false);
           return;
         }
-        // Defer to any Modal opened above the player (e.g. the download dialog): its Escape closes
-        // it, ours would otherwise also fire and close the player underneath it (U-C).
-        if (modalCount() > 0) return;
         onClose();
         return;
       }