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/ShareChooser.tsx

@@ -0,0 +1,119 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import { useRouter } from "next/navigation";
+
+import { Button, Spinner } from "@/components/ui";
+import { useI18n } from "@/lib/i18n";
+import { setPendingShare, type PendingShareFile } from "@/lib/share-pending";
+import { openSettings } from "@/lib/ui-events";
+
+type Payload = { title?: string; text?: string; url?: string; files: PendingShareFile[] };
+
+/**
+ * The share-target chooser: shows what was shared and lets the user send it
+ * to New deck or Make my homework. Files are stashed in sessionStorage and
+ * picked up by the wizard on mount.
+ */
+export function ShareChooser({
+  id,
+  text,
+  url,
+  error,
+}: {
+  id?: string;
+  text?: string;
+  url?: string;
+  error?: string;
+}) {
+  const { t } = useI18n();
+  const router = useRouter();
+  const [payload, setPayload] = useState<Payload | null>(id ? null : { text, url, files: [] });
+  const [failed, setFailed] = useState(Boolean(error));
+
+  useEffect(() => {
+    if (!id) return;
+    let cancelled = false;
+    void (async () => {
+      try {
+        const response = await fetch(`/share-target?id=${encodeURIComponent(id)}`, { cache: "no-store" });
+        if (!response.ok) throw new Error("expired");
+        const data = (await response.json()) as Payload;
+        if (!cancelled) setPayload(data);
+      } catch {
+        if (!cancelled) setFailed(true);
+      }
+    })();
+    return () => {
+      cancelled = true;
+    };
+  }, [id]);
+
+  const combinedText = [payload?.text, payload?.url].filter(Boolean).join("\n\n").trim();
+  const fileCount = payload?.files.length ?? 0;
+
+  const go = (target: "/create" | "/homework") => {
+    setPendingShare({ text: combinedText || undefined, files: payload?.files ?? [] });
+    router.push(target);
+  };
+
+  if (failed) {
+    return (
+      <div className="card mx-auto max-w-md space-y-3 p-6 text-center">
+        <p className="text-3xl" aria-hidden>
+          🧊
+        </p>
+        <h1 className="text-xl font-extrabold text-[var(--text)]">{t(error === "large" ? "share.tooLarge" : "share.expired")}</h1>
+        <div className="flex justify-center gap-2 pt-1">
+          <Button onClick={() => router.push("/")}>{t("common.library")}</Button>
+          <Button variant="secondary" onClick={openSettings}>
+            {t("common.settings")}
+          </Button>
+        </div>
+      </div>
+    );
+  }
+
+  if (!payload) {
+    return (
+      <div className="flex items-center justify-center gap-3 py-16 text-sm font-semibold text-[var(--text-muted)]">
+        <Spinner className="size-5" /> {t("share.loading")}
+      </div>
+    );
+  }
+
+  return (
+    <div className="mx-auto max-w-md space-y-4">
+      <section className="space-y-1.5">
+        <h1 className="text-[26px] font-semibold leading-tight tracking-[-0.02em] text-[var(--text)]">
+          {t("share.title")}
+        </h1>
+        <p className="text-[14px] text-frost-600 dark:text-frost-300">{t("share.intro")}</p>
+      </section>
+
+      <div className="card space-y-2 p-4">
+        <p className="text-[11px] font-semibold uppercase tracking-wide text-[var(--text-subtle)]">{t("share.preview")}</p>
+        {fileCount > 0 ? (
+          <p className="text-[13px] font-medium text-[var(--text)]">
+            {fileCount === 1 ? payload.files[0].name : `${fileCount} files`}
+          </p>
+        ) : null}
+        {combinedText ? (
+          <p className="line-clamp-4 text-[13px] leading-relaxed text-[var(--text-muted)]">{combinedText}</p>
+        ) : null}
+        {fileCount === 0 && !combinedText ? (
+          <p className="text-[13px] text-[var(--text-subtle)]">{t("share.nothing")}</p>
+        ) : null}
+      </div>
+
+      <div className="grid gap-2">
+        <Button size="lg" className="touch-target w-full" onClick={() => go("/create")}>
+          {t("common.newDeck")}
+        </Button>
+        <Button size="lg" variant="secondary" className="touch-target w-full" onClick={() => go("/homework")}>
+          {t("homework.nav")}
+        </Button>
+      </div>
+    </div>
+  );
+}