Fix ordering giveaways, the stuck false label, add Regenerate deck

Ordering cards
- Generation prompt now forbids dates, years, times, step numbers and
  ordinals in ordering items (spec + hard rules), and the server-side
  validation drops any ordering card whose items still contain them
  instead of shipping a giveaway.

True/False
- The 'false ✕' swipe label was permanently visible on the card: its
  opacity curve was inverted, so the clamp at rest was 1. Flipped the
  range (and tightened both ranges for the elastic drag) — each label now
  only fades in while swiping in that direction.

Regenerate deck
- Results screen gains a Regenerate deck button: rebuilds the activities
  from the deck's stored source notes, replaces the deck in storage
  (attempts/best score reset) and starts the fresh run from slide 1.
- StudyFeed tracks the current deck so the header, export and messages use
  the regenerated title/activities.
- Fixed a spurious 0% attempt recorded when a regenerated deck swapped in:
  scroll anchoring held the old results position, so the feed briefly
  reported 5/5 with zero answers. The scroll now resets before the new
  slides render, overflow-anchor is disabled on the feed, and a run with
  no answers is never recorded.

Tests: new deck-fixes suite 15/15 (TF overlay at rest/drag, regenerate
with loading state + storage reset, ordering guard drop/keep); retry
14/14, deck chrome 15/15, pwa 16/16, final-ux 12/12, security 8/8,
contrast 20/20.

components/activities/TrueFalse.tsx

@@ -19,8 +19,13 @@ type TrueFalseActivity = Extract<Activity, { type: "true_false" }>;
 export function TrueFalse({ activity, onAnswered, onNext, isLast }: ActivityComponentProps<TrueFalseActivity>) {
   const [result, setResult] = useState<boolean | null>(null);
   const x = useMotionValue(0);
-  const trueOpacity = useTransform(x, [30, 110], [0, 1]);
-  const falseOpacity = useTransform(x, [-30, -110], [1, 0]);
+  // The drag is elastic, so the card moves less than the pointer — these
+  // ranges make each label reach full strength on a normal swipe.
+  const trueOpacity = useTransform(x, [20, 70], [0, 1]);
+  // Left drags go negative: at rest (0) this must be invisible, only fading
+  // in as the card is dragged left. The old range was inverted, so the
+  // "false ✕" label sat on the card permanently.
+  const falseOpacity = useTransform(x, [-70, -20], [1, 0]);
 
   const answer = (value: boolean) => {
     if (result !== null) return;