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/index.css

@@ -581,6 +581,16 @@ body {
   .reveal-idle {
     display: none;
   }
+  /* Sibling of `.reveal` for a MOUSE-ONLY hover-reveal: unlike `.reveal` (shown at rest on touch),
+     a `.reveal-hover` control is removed entirely on a device that can't hover, so it is never an
+     invisible tap/Tab target. Pair it with the site's own `opacity-0 group-hover:… group-focus-within:…`
+     (which still governs the hover + keyboard case). For a full-card/edge affordance whose action is
+     already reachable on touch another way — VideoCard's play overlay (tapping the card plays/resumes)
+     and the Carousel arrows (swiping scrolls the strip) — where a permanent scrim/edge button is not
+     wanted. Group-agnostic (encodes only the touch removal), so it works under any `.group`/`group\/…`. */
+  .reveal-hover {
+    display: none;
+  }
 }
 
 /* Dark mode: the ambient accent pools are faint over a near-black bg, so the docked frosted