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.

lib/sync.ts

@@ -16,6 +16,7 @@ import { backupDataSchema } from "./backup";
 import { ApiError } from "./client-api";
 import { replaceFolders } from "./folders";
 import { replaceHomework } from "./homework";
+import { ensureLearningHydrated, learningStore, replaceLearning } from "./history";
 import { localeHeader, t } from "./i18n";
 import { authHeaders } from "./settings";
 import { deckStore, replaceDecks } from "./storage";
@@ -159,7 +160,8 @@ async function apiDelete(id: string): Promise<void> {
 /* Payload                                                             */
 /* ------------------------------------------------------------------ */
 
-function buildPayload(): SyncPayload {
+async function buildPayload(): Promise<SyncPayload> {
+  await ensureLearningHydrated();
   return {
     app: "freezmo",
     kind: "sync",
@@ -169,6 +171,8 @@ function buildPayload(): SyncPayload {
       decks: deckStore.getSnapshot(),
       folders: folderStore.getSnapshot(),
       homework: homeworkStore.getSnapshot(),
+      history: learningStore.getAnswersSnapshot(),
+      srs: learningStore.getSrsSnapshot(),
     },
   };
 }
@@ -182,6 +186,7 @@ async function applyRemote(key: CryptoKey, remote: { updatedAt: number; envelope
     replaceDecks(parsed.data.decks);
     replaceHomework(parsed.data.homework);
   });
+  await replaceLearning(parsed.data.history ?? [], parsed.data.srs ?? []);
   setSyncRecord({ lastSyncedAt: remote.updatedAt, dirtyAt: null, conflict: false });
 }
 
@@ -218,7 +223,7 @@ export async function syncNow(strategy: "auto" | "push" | "pull" = "auto"): Prom
       return { status: "up-to-date" };
     }
 
-    const { iv, ciphertext } = await encryptWithKey(key, buildPayload());
+    const { iv, ciphertext } = await encryptWithKey(key, await buildPayload());
     const envelope: SyncEnvelope = {
       v: 1,
       kdf: KDF,