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/Toaster.tsx

@@ -0,0 +1,67 @@
+"use client";
+
+import { AnimatePresence, motion } from "framer-motion";
+import { useEffect, useState } from "react";
+
+import { Button } from "@/components/ui";
+import { springGentle } from "@/lib/motion";
+import { toastBus, type Toast } from "@/lib/toast";
+
+/**
+ * Global toast host: undo for reversible destructive actions, share fallbacks.
+ * Sits above the tab bar on phones and near the bottom on desktop.
+ */
+export function Toaster() {
+  const [toasts, setToasts] = useState<Toast[]>([]);
+
+  useEffect(
+    () =>
+      toastBus.subscribe((toast) => {
+        setToasts((current) => [...current.slice(-2), toast]);
+        window.setTimeout(
+          () => setToasts((current) => current.filter((entry) => entry.id !== toast.id)),
+          toast.durationMs ?? 5000,
+        );
+      }),
+    [],
+  );
+
+  const dismiss = (id: number) => setToasts((current) => current.filter((entry) => entry.id !== id));
+
+  return (
+    <div className="pointer-events-none fixed inset-x-0 bottom-[calc(env(safe-area-inset-bottom,0px)+5.25rem)] z-[60] flex flex-col items-center gap-2 px-4 sm:bottom-6">
+      <AnimatePresence initial={false}>
+        {toasts.map((toast) => (
+          <motion.div
+            key={toast.id}
+            layout
+            initial={{ opacity: 0, y: 16 }}
+            animate={{ opacity: 1, y: 0 }}
+            exit={{ opacity: 0, y: 10 }}
+            transition={springGentle}
+            role="status"
+            className="pointer-events-auto flex w-full max-w-md items-center gap-3 rounded-2xl border border-[var(--glass-border)] bg-[color-mix(in_oklab,var(--surface)_92%,transparent)] px-4 py-3 shadow-lift backdrop-blur-xl"
+          >
+            <p className="min-w-0 flex-1 text-[13px] font-medium text-[var(--text)]">{toast.message}</p>
+            {toast.actionLabel ? (
+              <button
+                type="button"
+                onClick={() => {
+                  toast.onAction?.();
+                  dismiss(toast.id);
+                }}
+                className="shrink-0 text-[13px] font-bold text-accent-700 dark:text-accent-200"
+              >
+                {toast.actionLabel}
+              </button>
+            ) : (
+              <Button size="sm" variant="ghost" className="shrink-0" onClick={() => dismiss(toast.id)} aria-label="Dismiss">
+                ✕
+              </Button>
+            )}
+          </motion.div>
+        ))}
+      </AnimatePresence>
+    </div>
+  );
+}