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.

lib/share.ts

@@ -0,0 +1,72 @@
+"use client";
+
+import { exportDeck } from "./storage";
+import { showToast } from "./toast";
+import type { Deck, HomeworkRecord } from "./schemas";
+import { t } from "./i18n";
+
+/**
+ * Web Share with graceful fallbacks: file share (deck JSON) where the browser
+ * supports it, text share otherwise, clipboard copy as the last resort.
+ */
+
+type ShareNavigator = Navigator & {
+  canShare?: (data: { files?: File[]; text?: string; title?: string }) => boolean;
+  share?: (data: { files?: File[]; text?: string; title?: string }) => Promise<void>;
+};
+
+function deckFile(deck: Deck): File {
+  return new File([exportDeck(deck)], `${deck.title.replace(/[^\p{L}\p{N} _-]/gu, "").trim() || "deck"}.json`, {
+    type: "application/json",
+  });
+}
+
+async function copyText(text: string): Promise<boolean> {
+  try {
+    if (navigator.clipboard?.writeText) {
+      await navigator.clipboard.writeText(text);
+      return true;
+    }
+  } catch {
+    /* fall through */
+  }
+  return false;
+}
+
+export async function shareDeck(deck: Deck): Promise<void> {
+  const nav = navigator as ShareNavigator;
+  const file = deckFile(deck);
+  try {
+    if (nav.share && nav.canShare?.({ files: [file] })) {
+      await nav.share({ files: [file], title: deck.title });
+      return;
+    }
+    if (nav.share) {
+      await nav.share({ title: deck.title, text: deck.title });
+      return;
+    }
+  } catch (error) {
+    // The user cancelling the share sheet is not an error.
+    if (error instanceof DOMException && error.name === "AbortError") return;
+  }
+  if (await copyText(exportDeck(deck))) showToast({ message: t("share.copied") });
+  else showToast({ message: t("share.failed") });
+}
+
+export async function shareHomework(record: HomeworkRecord): Promise<void> {
+  const nav = navigator as ShareNavigator;
+  const text =
+    record.mode === "make"
+      ? record.answer
+      : record.steps.map((step, index) => `${index + 1}. ${step.title}`).join("\n");
+  try {
+    if (nav.share) {
+      await nav.share({ title: record.title, text });
+      return;
+    }
+  } catch (error) {
+    if (error instanceof DOMException && error.name === "AbortError") return;
+  }
+  if (await copyText(text)) showToast({ message: t("share.copied") });
+  else showToast({ message: t("share.failed") });
+}