Native-feel pass: PWA integration, system back, offline, haptics, undo

Mobile only (<=639px / coarse pointers); desktop keeps its behaviour.

System integration
- Manifest: id, display_override, monochrome icon, Make my homework
  shortcut and a share target (text/URL via GET, files via POST).
  iOS startup images for the common iPhone sizes. Pinch-zoom is allowed
  again (double-tap zoom is disabled via touch-action instead).
- Share target: /share chooser with New deck / Make my homework; POSTed
  files wait in a 10-minute in-memory store, reach the wizard through
  sessionStorage and never touch disk.
- Web Share: Share rows for decks (JSON file where supported) and the
  homework result, with a clipboard fallback and a toast.
- Offline: a quiet pill appears and Freeze / mode cards are disabled
  with a clear reason; input is never lost (error cards keep Retry).
- State: wizard drafts (text, options, photos up to a size budget)
  survive reloads and OS kills and are cleared on success; re-tapping
  the active tab scrolls to the top; the deck page shows a skeleton
  instead of a spinner.

Navigation and gestures
- System Back closes the topmost layer first: a small history-entry
  stack (lib/use-back-close.ts) covers the drawer, Settings sheet,
  install sheet, deck actions panel and one wizard step; navigations
  are never rewound and stale entries are skipped.
- Destructive-but-reversible actions use Undo toasts: deleting a deck
  (no confirm) and removing a file in the wizard. Irreversible ones
  keep their confirm.

Input and feedback
- Every phone input got inputmode / enterkeyhint / autocomplete /
  autocapitalize / spellcheck; Enter on the last field advances the
  wizard; a clipboard-paste button appears where the API exists.
- Haptics behind a Settings toggle (on by default): tab switches,
  toggles, correct/wrong, completion; Vibration API + the iOS hidden
  switch tick, throttled and silent everywhere else.
- Wake lock during the study feed and homework guide, released on
  leave and re-acquired when the tab returns.

Verified: new native suite 35/35 (manifest/head/viewport, offline,
share target GET + POST, back-gesture layers, undo, share fallback,
tab re-tap, haptics persistence, draft restore), regression 74/74,
mobile 52/52, wizard 80/80, declutter 116/116, animation 8/8,
i18n 25/25. Lint + build clean. SW freezmo-v23.

Needs a real device (documented in the README): iOS haptic tick,
system share sheet entry, wake-lock grant, iPhone startup images,
iOS edge-swipe back, install prompts. Pull-to-refresh and the app
badge were skipped on purpose.

components/StudyFeed.tsx

@@ -8,9 +8,11 @@ import { ActivityRenderer } from "@/components/activities";
 import { celebrate } from "@/lib/confetti";
 import { ApiError, generateDeck } from "@/lib/client-api";
 import { materialiseActivities } from "@/lib/deck-ops";
+import { haptic } from "@/lib/haptics";
 import { useI18n } from "@/lib/i18n";
 import type { Activity, Deck } from "@/lib/schemas";
 import { recordAttempt, upsertDeck } from "@/lib/storage";
+import { useWakeLock } from "@/lib/use-wake-lock";
 import { cn } from "@/lib/utils";
 import { BoltIcon, FlameIcon, XIcon } from "@/components/icons";
 import { Button, ProgressBar } from "@/components/ui";
@@ -21,6 +23,8 @@ export type AttemptResult = { correct: boolean; userAnswer?: string };
 
 export function StudyFeed({ deck }: { deck: Deck }) {
   const { t } = useI18n();
+  /* Keep the screen awake for the whole study session (feature-detected). */
+  useWakeLock(true);
   const [currentDeck, setCurrentDeck] = useState(deck);
   const [queue, setQueue] = useState<Activity[]>(deck.activities);
   const [index, setIndex] = useState(0);
@@ -108,6 +112,7 @@ export function StudyFeed({ deck }: { deck: Deck }) {
       // 0% attempt before the feed had snapped back to the first slide.
       if (answered.length > 0) {
         recordAttempt(currentDeck.id, { scorePercent: score, correct, total });
+        haptic("celebrate");
         void celebrate("big");
       }
     }
@@ -120,12 +125,14 @@ export function StudyFeed({ deck }: { deck: Deck }) {
         return { ...current, [activity.id]: { correct, userAnswer: meta?.userAnswer } };
       });
       if (correct) {
+        haptic("success");
         const nextStreak = streak + 1;
         setStreak(nextStreak);
         setBestStreak((best) => Math.max(best, nextStreak));
         setXp((value) => value + activity.xp);
         if (nextStreak > 0 && nextStreak % 5 === 0) void celebrate("normal");
       } else {
+        haptic("error");
         setStreak(0);
       }
     },