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.

components/ReviewClient.tsx

@@ -0,0 +1,117 @@
+"use client";
+
+import { useEffect, useMemo, useState, useSyncExternalStore } from "react";
+import { useRouter } from "next/navigation";
+
+import { SessionFeed, type SessionItem } from "@/components/SessionFeed";
+import { Button } from "@/components/ui";
+import { haptic } from "@/lib/haptics";
+import { ensureLearningHydrated, learningStore, recordAnswer } from "@/lib/history";
+import { useI18n } from "@/lib/i18n";
+import { dueItems, mistakeItems, type ReviewItem } from "@/lib/learning";
+
+/**
+ * Review / mistakes session: built from due items (SRS) across every deck, or
+ * from the activities whose latest answer was wrong. Every answer is recorded,
+ * so mistakes clear themselves once answered correctly.
+ */
+export function ReviewClient({ mistakes, deckId }: { mistakes: boolean; deckId?: string }) {
+  const { t } = useI18n();
+  const router = useRouter();
+  const answers = useSyncExternalStore(
+    learningStore.subscribe,
+    learningStore.getAnswersSnapshot,
+    learningStore.getServerAnswers,
+  );
+  const srs = useSyncExternalStore(
+    learningStore.subscribe,
+    learningStore.getSrsSnapshot,
+    learningStore.getServerSrs,
+  );
+  const [ready, setReady] = useState(false);
+  /** Frozen at session start: answering must not shrink the live queue. */
+  const [session, setSession] = useState<ReviewItem[] | null>(null);
+
+  useEffect(() => {
+    void ensureLearningHydrated().then(() => setReady(true));
+  }, []);
+
+  const items = useMemo(
+    () => (mistakes ? mistakeItems(answers, deckId) : dueItems(srs)),
+    [mistakes, deckId, answers, srs],
+  );
+
+  useEffect(() => {
+    if (!ready || session || items.length === 0) return;
+    setSession(items);
+  }, [ready, session, items]);
+
+  const title = mistakes
+    ? deckId
+      ? t("review.mistakesDeck")
+      : t("review.mistakesTitle")
+    : t("review.title");
+
+  if (!ready) {
+    return (
+      <main className="frost-bg grid min-h-dvh place-items-center px-4">
+        <div className="w-full max-w-xl space-y-4" aria-busy="true" aria-label={title}>
+          <div className="skeleton h-24 rounded-3xl" />
+          <div className="skeleton h-12 rounded-2xl" />
+        </div>
+      </main>
+    );
+  }
+
+  if (!session) {
+    return (
+      <main className="frost-bg grid min-h-dvh place-items-center px-4">
+        <div className="card max-w-md space-y-3 p-6 text-center">
+          <h1 className="text-xl font-extrabold text-[var(--text)]">{title}</h1>
+          <p className="text-sm leading-relaxed text-[var(--text-muted)]">
+            {mistakes ? t("review.noMistakes") : t("review.nothingDue")}
+          </p>
+          <Button size="lg" className="touch-target" onClick={() => router.push("/")}>
+            {t("review.done")}
+          </Button>
+        </div>
+      </main>
+    );
+  }
+
+  const sessionItems: SessionItem[] = session.map(({ deck, activity }) => ({ deck, activity }));
+
+  return (
+    <SessionFeed
+      title={title}
+      items={sessionItems}
+      testId="review-feed"
+      onExit={() => router.push("/")}
+      onAnswered={(item, correct) => {
+        haptic(correct ? "success" : "error");
+        recordAnswer({
+          deckId: item.deck.id,
+          deckTitle: item.deck.title,
+          activityId: item.activity.id,
+          type: item.activity.type,
+          topic: item.activity.topic,
+          correct,
+        });
+      }}
+      renderResults={({ correct, total, score }) => (
+        <div className="card space-y-4 p-5 text-center">
+          <p className="text-[11px] font-semibold uppercase tracking-wider text-[var(--text-subtle)]">{title}</p>
+          <p className="text-[42px] font-extrabold leading-none text-[var(--text)]">{score}%</p>
+          <p className="text-sm font-medium text-[var(--text-muted)]">
+            {t("results.gotOf", { correct, total, retry: "" })}
+          </p>
+          <div className="flex flex-wrap justify-center gap-2">
+            <Button size="lg" className="touch-target" onClick={() => router.push("/")}>
+              {t("review.done")}
+            </Button>
+          </div>
+        </div>
+      )}
+    />
+  );
+}