Phase 2: learning engine — history, SRS review, mistakes, stats, exam, reminders

Answer history and schedules
- Every answer is recorded (deck, activity, topic, correct/wrong, time)
  in a versioned IndexedDB log (freezmo-learning v1, stores answers +
  srs), mirrored in memory for synchronous React reads and capped at
  20k answers (oldest pruned). StudyFeed records as it grades.
- Light SM-2 schedule per activity: 1 day, 3 days, then interval x
  ease; wrong answers return in ten minutes with the ease nudged down.

Review, mistakes, stats
- Library 'Smart review' card: due count, mistakes count (latest
  answer wrong — clears itself when answered correctly), today's goal
  progress. Review sessions freeze their item list at start.
- New /review (due across decks, ?mistakes=1 global or ?deck= per
  deck) built on a shared SessionFeed with the study feed's snap/keyboard
  mechanics, wake lock and a results slide.
- New /stats: day streak, five-week calendar, accuracy per subject and
  per deck, weakest topics (>=3 answers), daily-goal progress.
- Daily goal configurable in Settings (10/20/30/50).

Exam mode
- New /exam: pick decks, question count, optional timer; no hints
  (a NoHints context hides hint rows and the self-check escape);
  results with score, per-deck and per-topic breakdown; wrong answers
  land in the mistakes set through the normal history.

Reminders (conditional)
- VAPID-gated: GET/POST/DELETE /api/push and POST /api/push/test with
  the standard route pattern, RATE_LIMIT_PUSH, zod, auth before body.
  Subscriptions stored server-side as endpoint + keys only (PUSH_DIR).
  UI hidden entirely without keys; iOS install note; service worker
  push/notificationclick handlers; scripts/send-reminders.mjs for a
  cron/systemd timer; web-push added as the one server dependency.

Verified: new phase-2 suite 42/42 (history/SRS shapes from a real study
run, mistakes clearing, review session, stats math, exam timer and
breakdowns, VAPID-less hidden state plus the configured
subscribe/validate/test/delete path against a second instance) plus
regression 74/74, mobile 52/52, wizard 80/80, declutter 116/116,
animation 8/8, native 35/35, phase1 38/38, i18n 25/25. Key parity
709/709. Lint + build clean. SW freezmo-v26.

scripts/send-reminders.mjs

@@ -0,0 +1,67 @@
+/**
+ * Sends the daily review reminder to every stored push subscription.
+ *
+ * Meant for cron or a systemd timer, e.g.:
+ *   0 18 * * *  cd /path/to/freezmo && node scripts/send-reminders.mjs
+ *
+ * Silently does nothing when VAPID keys are not configured (the whole
+ * reminder feature stays hidden in the UI in that case).
+ */
+import { readFile, rename, writeFile } from "node:fs/promises";
+import path from "node:path";
+import webpush from "web-push";
+
+const dir = process.env.PUSH_DIR?.trim() || path.join(process.cwd(), ".freezmo", "push");
+const publicKey = process.env.VAPID_PUBLIC_KEY?.trim();
+const privateKey = process.env.VAPID_PRIVATE_KEY?.trim();
+const subject = process.env.VAPID_SUBJECT?.trim() || "mailto:freezmo@localhost";
+const title = process.env.PUSH_REMINDER_TITLE?.trim() || "Time to review";
+const body =
+  process.env.PUSH_REMINDER_BODY?.trim() ||
+  "Your decks are waiting — a few due cards keep the streak alive.";
+
+if (!publicKey || !privateKey) {
+  console.log("[freezmo] VAPID keys are not configured — nothing to send.");
+  process.exit(0);
+}
+
+const file = path.join(dir, "subscriptions.json");
+let subscriptions = [];
+try {
+  subscriptions = JSON.parse(await readFile(file, "utf8"));
+} catch {
+  subscriptions = [];
+}
+
+if (!Array.isArray(subscriptions) || subscriptions.length === 0) {
+  console.log("[freezmo] no push subscriptions stored.");
+  process.exit(0);
+}
+
+webpush.setVapidDetails(subject, publicKey, privateKey);
+
+let sent = 0;
+let removed = 0;
+const remaining = [];
+for (const entry of subscriptions) {
+  try {
+    await webpush.sendNotification(
+      { endpoint: entry.endpoint, keys: entry.keys },
+      JSON.stringify({ title, body, url: "/" }),
+    );
+    sent += 1;
+    remaining.push(entry);
+  } catch (error) {
+    const status = error?.statusCode;
+    if (status === 404 || status === 410) removed += 1;
+    else remaining.push(entry);
+  }
+}
+
+if (removed > 0) {
+  const temporary = `${file}.${Date.now()}.tmp`;
+  await writeFile(temporary, JSON.stringify(remaining, null, 2), "utf8");
+  await rename(temporary, file);
+}
+
+console.log(`[freezmo] reminders sent: ${sent}, dead subscriptions removed: ${removed}`);