Smooth screen transitions: new screen waits hidden until its data arrives, no half-built flash

- Navigator stage: the next screen mounts hidden under the current one and is revealed only when its
  first data has arrived (ScreenGate held by useAsync/useGateHold), at most 450 ms; a thin top bar shows
  after 120 ms of waiting. Push slides from the left, back from the right, tabs only fade.
- Session cache (stale-while-revalidate) for screen GETs: revisited screens draw instantly.
- Home map and stats are fetched during the splash, so the first screen arrives complete.
- Entrance pop-ins (mascot, islands, lesson cards) run on first view only, not on every revisit.
- Shop status card shows placeholders instead of a wrong «نداری» while loading.
- Re-tapping the open tab scrolls to top instead of reloading; sheets close with a swipe down.
- Quiz: «۱ از ۴» instead of «۱/۴» (read as a fraction), varied praise, «N تا پشتِ هم درست»,
  growth-mindset copy after a weak round, zero-reward tiles hidden.
- Streak: a concrete next step when the streak is 0 or today is still empty.

Checked in headless Chrome against the local backend (frame captures at 150 ms and 700 ms latency,
rapid tab tapping, full quiz run). Not checked on a real phone or iOS Safari.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/screens/LessonScreen.tsx

@@ -15,7 +15,7 @@ import { Diagram } from '@/diagram';
 export function LessonScreen({ lessonId, nodeId }: { lessonId: number; nodeId?: string }) {
   const { home, companion } = useAppData();
   const nav = useNav();
-  const intro = useAsync(() => api.lesson.intro(lessonId), [lessonId]);
+  const intro = useAsync(() => api.lesson.intro(lessonId), [lessonId], { key: `lesson-intro:${lessonId}` });
   const ref = findNode(home, nodeId) ?? findNodeOfLesson(home, lessonId);
   const color = ref ? unitColor(ref.unitIndex) : '#ED6956';
   const data = intro.data;