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/Carousel.tsx

@@ -77,8 +77,10 @@ export function Carousel({
 
   const label = ariaLabel ?? (typeof title === "string" ? title : undefined);
 
+  // Mouse-only: hidden until the carousel is hovered or the arrow is keyboard-focused; `reveal-hover`
+  // removes it on touch, where the strip scrolls by swipe — so an invisible edge button never eats a tap.
   const arrowBtn =
-    "absolute top-1/2 -translate-y-1/2 z-10 grid h-9 w-9 place-items-center rounded-full " +
+    "reveal-hover absolute top-1/2 -translate-y-1/2 z-10 grid h-9 w-9 place-items-center rounded-full " +
     "bg-black/55 text-white backdrop-blur-xs border border-white/15 shadow-lg transition " +
     "opacity-0 group-hover/car:opacity-100 focus-visible:opacity-100 hover:bg-black/75";