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.

public/sw.js

@@ -9,7 +9,7 @@
  *
  * Bumping CACHE_VERSION is how a new deploy invalidates old assets.
  */
-const CACHE_VERSION = "freezmo-v25";
+const CACHE_VERSION = "freezmo-v26";
 const SHELL_CACHE = `${CACHE_VERSION}-shell`;
 const ASSET_CACHE = `${CACHE_VERSION}-assets`;
 
@@ -53,6 +53,43 @@ function isCacheableAsset(url) {
   );
 }
 
+/* ------------------------------------------------------------------ */
+/* Push reminders (opt-in; only active when VAPID is configured)       */
+/* ------------------------------------------------------------------ */
+
+self.addEventListener("push", (event) => {
+  let data = {};
+  try {
+    data = event.data ? event.data.json() : {};
+  } catch {
+    data = {};
+  }
+  event.waitUntil(
+    self.registration.showNotification(data.title || "Freezmo", {
+      body: data.body || "",
+      icon: "/icons/icon-192.png",
+      badge: "/icons/icon-192.png",
+      data: { url: data.url || "/" },
+    }),
+  );
+});
+
+self.addEventListener("notificationclick", (event) => {
+  event.notification.close();
+  const url = (event.notification.data && event.notification.data.url) || "/";
+  event.waitUntil(
+    self.clients.matchAll({ type: "window", includeUncontrolled: true }).then((clients) => {
+      for (const client of clients) {
+        if ("focus" in client) {
+          client.navigate(url).catch(() => {});
+          return client.focus();
+        }
+      }
+      return self.clients.openWindow(url);
+    }),
+  );
+});
+
 self.addEventListener("fetch", (event) => {
   const request = event.request;
   if (request.method !== "GET") return;