fix(ui): a card's hover chrome answers keyboard focus, not any focus

Back from a channel page hands the card its focus again after a mouse trip;
revealed on focus-within, its hover row stayed up under no pointer until a
reload (owner). The row, the Play layer, the Continue + Restart pair and the
global .reveal rule now show on hover or on :focus-visible on the card or
inside it, so Tab and the arrow keys still reveal it.

frontend/src/components/VideoCard.tsx

@@ -295,7 +295,7 @@ function Thumb({
           </span>
         )}
         {/* 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 on hover AND on keyboard focus (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 && (