fix(player): Escape coordination, focused-Space, light-theme menus (U-C, U-3.1.4, U-3.3.12)

Three player/overlay interaction fixes (PlexPlayer changes share a file, hence one
commit):
- U-C (Escape double-close): useDismiss's Escape now stopPropagation()s (its document
  listener bubbles before a player's window listener, so one Escape closes only the
  popover, not the popover AND the player). Modal exports modalCount(); both players'
  Escape defers when a Modal is open above them, so Escape closes the dialog, not the
  player under it and then the dialog.
- U-3.1.4: PlexPlayer's Space now defers to a focused BUTTON/A (e.g. "Skip intro")
  instead of toggling playback — matching PlayerModal.
- U-3.3.12: the audio/subtitle menus used theme-adaptive glass-menu + forced
  text-white (white-on-white in light theme); they now use the player's own explicit
  dark panel (border-white/15 + bg-neutral-900/95), readable in both themes.

frontend/src/components/Modal.tsx

@@ -8,6 +8,13 @@ import { useBackToClose } from "../lib/history";
 let modalStack: number[] = [];
 let nextModalId = 1;
 
+/** How many <Modal>s are currently open. The players (which use their own window-level Escape
+ * listeners, not this stack) check it so their Escape defers to a Modal opened above them —
+ * otherwise one Escape closes the player UNDER the dialog and then the dialog too (U-C). */
+export function modalCount(): number {
+  return modalStack.length;
+}
+
 // Small centered modal shell (portaled to <body>): backdrop + ESC + scroll-lock close.
 export default function Modal({
   title,