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/OfflineBanner.tsx

@@ -0,0 +1,32 @@
+"use client";
+
+import { AnimatePresence, motion } from "framer-motion";
+
+import { useI18n } from "@/lib/i18n";
+import { fade } from "@/lib/motion";
+import { useOnline } from "@/lib/use-online";
+
+/** A quiet offline pill under the header; saved decks keep working offline. */
+export function OfflineBanner() {
+  const online = useOnline();
+  const { t } = useI18n();
+
+  return (
+    <AnimatePresence>
+      {!online ? (
+        <motion.div
+          initial={{ opacity: 0, y: -8 }}
+          animate={{ opacity: 1, y: 0 }}
+          exit={{ opacity: 0, y: -8 }}
+          transition={fade}
+          role="status"
+          className="pointer-events-none fixed inset-x-0 top-0 z-[60] flex justify-center px-4 pt-safe sm:hidden"
+        >
+          <p className="rounded-full border border-amber-300/70 bg-amber-50/95 px-3.5 py-1.5 text-[12px] font-semibold text-amber-900 shadow-lift backdrop-blur-xl dark:border-amber-400/30 dark:bg-amber-500/20 dark:text-amber-100">
+            {t("offline.banner")}
+          </p>
+        </motion.div>
+      ) : null}
+    </AnimatePresence>
+  );
+}