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/Flashcard.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { motion } from "framer-motion";
+import { motion, useReducedMotion } from "framer-motion";
 import { useState } from "react";
 
 import { useI18n } from "@/lib/i18n";
@@ -20,8 +20,21 @@ type FlashcardActivity = Extract<Activity, { type: "flashcard" }>;
 
 export function Flashcard({ activity, onAnswered, onNext, isLast, nextLabel }: ActivityComponentProps<FlashcardActivity>) {
   const { t } = useI18n();
+  const reduceMotion = useReducedMotion();
   const [flipped, setFlipped] = useState(false);
   const [result, setResult] = useState<boolean | null>(null);
+  /**
+   * Which face is painted. The swap happens at the 90° edge-on moment instead
+   * of relying on backface-visibility: a backdrop-filter layer (the card
+   * glass) keeps rendering its backface on some GPU/compositing paths, which
+   * showed the question mirrored over the answer.
+   */
+  const [face, setFace] = useState<"front" | "back">("front");
+
+  const showFaceFor = (angle: number) => {
+    const next = angle > 90 && angle < 270 ? "back" : "front";
+    setFace((current) => (current === next ? current : next));
+  };
 
   const settle = (correct: boolean) => {
     if (result !== null) return;
@@ -42,10 +55,15 @@ export function Flashcard({ activity, onAnswered, onNext, isLast, nextLabel }: A
       >
         <motion.div
           animate={{ rotateX: flipped ? 180 : 0 }}
-          transition={{ type: "spring", stiffness: 220, damping: 24 }}
+          transition={reduceMotion ? { duration: 0 } : { type: "spring", stiffness: 220, damping: 24 }}
+          onUpdate={(latest) => showFaceFor(Number(latest.rotateX ?? 0))}
+          onAnimationComplete={() => showFaceFor(flipped ? 180 : 0)}
           className="relative min-h-56 w-full [transform-style:preserve-3d]"
         >
-          <PromptCard className="absolute inset-0 flex flex-col justify-center gap-3 group-hover:border-frost-300 [backface-visibility:hidden]">
+          <PromptCard
+            className="absolute inset-0 flex flex-col justify-center gap-3 group-hover:border-frost-300"
+            style={{ visibility: face === "front" ? "visible" : "hidden" }}
+          >
             <span className="text-xs font-bold uppercase tracking-widest text-frost-600 dark:text-frost-200">
               {t("flashcard.question")}
             </span>
@@ -55,7 +73,7 @@ export function Flashcard({ activity, onAnswered, onNext, isLast, nextLabel }: A
             </span>
           </PromptCard>
 
-          <div className="[transform:rotateX(180deg)] [backface-visibility:hidden]">
+          <div className="[transform:rotateX(180deg)]" style={{ visibility: face === "back" ? "visible" : "hidden" }}>
             <div className="card flex min-h-56 flex-col justify-center gap-3 px-5 py-6 sm:px-6">
               <span className="text-xs font-bold uppercase tracking-widest text-frost-600 dark:text-frost-200">
                 {t("flashcard.answer")}

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>
   );
 }