fix(a11y): touch/keyboard reveal tail + single-Escape layer coordination (UX-7/UX-8)

UX-8: one Escape must close only the topmost layer, never both an open
popover AND the player beneath it. The popover's Escape listener is on
`document` and fires before the player's on `window`; closing the popover
flushes its layer off the shared stack synchronously, so the player's
`anyLayerAbove()` lookup read "nothing above" and closed too. Add a
consumed-event flag to layerStack (`markEscapeHandled`/`escapeHandled`):
the acting topmost layer stamps the shared KeyboardEvent, and both players
check the stamp before the (now-mutated) stack. Immune to listener order
and mid-dispatch stack mutation. Cover with layerStack.dom.test.ts.

UX-7: the E4 `.reveal` sweep already covered the card action rows; two
residual hover-only traps remained — invisible-but-tappable/Tab-focusable
on touch. Add a group-agnostic `.reveal-hover` recipe (display:none under
`(hover: none)`) paired with each site's own hover/focus-within reveal:
- VideoCard play/Continue/Restart overlay: also reveal on
  group-focus-within (keyboard), and drop it on touch — the card tap
  already plays/resumes, so no permanent scrim over every mobile thumbnail.
- Carousel prev/next arrows: gone on touch, where the strip swipes.

frontend/src/components/VideoCard.tsx

@@ -262,9 +262,12 @@ function Thumb({
           <Bookmark className="w-3.5 h-3.5" />
         </span>
       )}
-      {/* Hover overlay: Play everywhere; Continue + Restart once the video is in progress. */}
+      {/* Hover overlay: Play everywhere; Continue + Restart once the video is in progress. Mouse-only:
+          reveal on hover AND on keyboard focus-within (else a Tab-focused button is invisible), and
+          `reveal-hover` drops it entirely on touch — the card tap already plays/resumes there, so a
+          permanent dark scrim over every mobile thumbnail (and an invisible tap target) is not wanted. */}
       {onOpen && (
-        <div className="absolute inset-0 flex items-center justify-center gap-2 bg-black/45 opacity-0 group-hover:opacity-100 transition-opacity">
+        <div className="absolute inset-0 flex items-center justify-center gap-2 bg-black/45 opacity-0 group-hover:opacity-100 group-focus-within:opacity-100 transition-opacity reveal-hover">
           {inProgress ? (
             <>
               <button