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/haptics.ts

@@ -0,0 +1,63 @@
+"use client";
+
+import { settingsStore } from "./settings";
+
+/**
+ * Best-effort haptics, behind the Settings → Haptics toggle (on by default).
+ *
+ * Android/Chrome use the Vibration API. iOS ignores `navigator.vibrate`;
+ * since iOS 17.4 the system tick can be triggered by clicking a hidden
+ * `<input type="checkbox" switch>` label — we try that, throttled, and fail
+ * silently everywhere else. Nothing here ever throws or blocks input.
+ */
+
+export type HapticKind = "tap" | "toggle" | "success" | "error" | "celebrate";
+
+const PATTERNS: Record<HapticKind, number | number[]> = {
+  tap: 8,
+  toggle: 12,
+  success: [14, 40, 18],
+  error: [30, 60, 30],
+  celebrate: [18, 50, 18, 50, 26],
+};
+
+let lastIosTick = 0;
+
+function iosTick(): void {
+  const now = Date.now();
+  if (now - lastIosTick < 80) return;
+  lastIosTick = now;
+  try {
+    const label = document.createElement("label");
+    label.setAttribute("aria-hidden", "true");
+    label.style.position = "fixed";
+    label.style.left = "-9999px";
+    const input = document.createElement("input");
+    input.type = "checkbox";
+    input.setAttribute("switch", "");
+    label.appendChild(input);
+    document.body.appendChild(label);
+    input.click();
+    window.setTimeout(() => label.remove(), 0);
+  } catch {
+    /* no haptics available */
+  }
+}
+
+export function haptic(kind: HapticKind = "tap"): void {
+  if (typeof window === "undefined") return;
+  try {
+    if (settingsStore.getSnapshot().haptics === false) return;
+  } catch {
+    return;
+  }
+  try {
+    if (typeof navigator !== "undefined" && typeof navigator.vibrate === "function") {
+      navigator.vibrate(PATTERNS[kind]);
+      return;
+    }
+    iosTick();
+  } catch {
+    /* silent */
+  }
+}