Flashcard flip: paint one face explicitly, never rely on backface-visibility

On a GPU-composited desktop the flipped card showed the question mirrored
over the answer. Both faces are glass layers with backdrop-filter, and on
some compositing paths the front's backface keeps rendering, so the
pre-rotated back face and the mirrored front face both painted.

- The faces are now painted by an explicit visibility swap driven by the
  animation's own rotation (onUpdate): the swap lands at the 90° edge-on
  moment, where it cannot be seen, so exactly one face is ever in the
  render tree — independent of backface-visibility and of the browser's
  3D flattening rules. onAnimationComplete lands the swap for instant
  flips (reduced motion now skips the spring, matching the app's motion
  rules).
- PromptCard takes an optional style so the face can carry the visibility.
- phase-2 suite: the painted-face invariant at rest, flipped and flipped
  back, plus the answer reading inside the card (56 checks).
- README: decision 53. SW -> freezmo-v39.

components/activities/shared.tsx

@@ -70,12 +70,19 @@ export function ActivityHeader({
 export function PromptCard({
   children,
   className,
+  style,
 }: {
   children: React.ReactNode;
   className?: string;
+  style?: React.CSSProperties;
 }) {
   return (
-    <div className={cn("glass-panel glass-solid rounded-[1.5rem] px-5 py-6 text-balance sm:px-6", className)}>{children}</div>
+    <div
+      className={cn("glass-panel glass-solid rounded-[1.5rem] px-5 py-6 text-balance sm:px-6", className)}
+      style={style}
+    >
+      {children}
+    </div>
   );
 }