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/homework/HomeworkGuide.tsx

@@ -10,6 +10,7 @@ import { Button, ProgressBar } from "@/components/ui";
 import { assembleSolution } from "@/lib/homework";
 import { useI18n } from "@/lib/i18n";
 import type { HomeworkGuideRecord, HomeworkStep } from "@/lib/schemas";
+import { useWakeLock } from "@/lib/use-wake-lock";
 
 import { CopyButton } from "./CopyButton";
 
@@ -33,6 +34,8 @@ export function HomeworkGuide({
   againLoading: boolean;
 }) {
   const { t } = useI18n();
+  /* Keep the screen awake while working through the guide. */
+  useWakeLock(true);
   const steps = record.steps;
   const slideCount = steps.length + 1; // + the solution slide
   const [index, setIndex] = useState(0);