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/use-wake-lock.ts

@@ -0,0 +1,51 @@
+"use client";
+
+import { useEffect } from "react";
+
+type WakeLockSentinelLike = {
+  release: () => Promise<void>;
+  addEventListener: (type: string, listener: () => void) => void;
+};
+
+type NavigatorWithWakeLock = Navigator & {
+  wakeLock?: { request: (type: "screen") => Promise<WakeLockSentinelLike> };
+};
+
+/**
+ * Keeps the screen awake while a study session or guide is open
+ * (feature-detected; silently does nothing where unsupported or denied).
+ * Re-acquires when the tab becomes visible again, releases on leave.
+ */
+export function useWakeLock(active = true): void {
+  useEffect(() => {
+    if (!active || typeof navigator === "undefined") return;
+    const wakeLock = (navigator as NavigatorWithWakeLock).wakeLock;
+    if (!wakeLock) return;
+
+    let sentinel: WakeLockSentinelLike | null = null;
+    let disposed = false;
+
+    const acquire = async () => {
+      try {
+        sentinel = await wakeLock.request("screen");
+        sentinel.addEventListener("release", () => {
+          sentinel = null;
+        });
+      } catch {
+        /* denied or not allowed without a gesture — fine */
+      }
+    };
+
+    const onVisibility = () => {
+      if (document.visibilityState === "visible" && !sentinel && !disposed) void acquire();
+    };
+
+    void acquire();
+    document.addEventListener("visibilitychange", onVisibility);
+    return () => {
+      disposed = true;
+      document.removeEventListener("visibilitychange", onVisibility);
+      void sentinel?.release().catch(() => {});
+    };
+  }, [active]);
+}