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.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v38";
+const CACHE_VERSION = "freezmo-v39";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;