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/PlexPlayer.tsx

@@ -46,7 +46,7 @@ import { LS, useAccountPersistedObject } from "../lib/storage";
 import { useDismiss } from "../lib/useDismiss";
 import { useScrollFade } from "../lib/useScrollFade";
 import { useBackToClose } from "../lib/history";
-import { modalCount } from "./Modal";
+import { pushLayer, removeLayer, anyLayerAbove } from "../lib/layerStack";
 
 // The rich info overlay (poster/cast/ratings) reuses the same component as the card's info page.
 const PlexInfo = lazy(() => import("./PlexInfo"));
@@ -808,6 +808,18 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
     handleBack();
   }, [handleBack]);
 
+  // This player is a dismissable layer: register it so a Modal or popover opened ABOVE it (its own
+  // settings/tracks menus included) takes Escape first and this player defers (R8 S3).
+  const layerIdRef = useRef<number | null>(null);
+  useEffect(() => {
+    const id = pushLayer();
+    layerIdRef.current = id;
+    return () => {
+      removeLayer(id);
+      layerIdRef.current = null;
+    };
+  }, []);
+
   // --- keyboard --------------------------------------------------------------------------------
   useEffect(() => {
     const onKey = (e: KeyboardEvent) => {
@@ -874,8 +886,10 @@ export default function PlexPlayer({ itemId, onClose, queue }: Props) {
           else handleBack();
           break;
         case "Escape":
-          // Defer to any Modal opened above the player — let its Escape close it, not ours (U-C).
-          if (modalCount() > 0) break;
+          // Defer to any layer opened above the player (Modal or anchored popover) — let it take the
+          // Escape, not us. A player MENU registers as a layer too, so it closes first and the next
+          // Escape reaches the auto-skip-cancel below (the old blanket-stopPropagation bug).
+          if (layerIdRef.current != null && anyLayerAbove(layerIdRef.current)) break;
           if (skipProgressRef.current != null) cancelAutoSkipRef.current();
           else if (menuOpenRef.current) {
             setMenuOpen(false);