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/backup.ts

@@ -14,8 +14,9 @@ import { z } from "zod";
 import { applyTheme } from "@/components/ThemeToggle";
 import { replaceFolders, folderStore } from "./folders";
 import { homeworkStore, replaceHomework } from "./homework";
+import { ensureLearningHydrated, learningStore, replaceLearning } from "./history";
 import { setLocale, t } from "./i18n";
-import { folderSchema, deckSchema, homeworkRecordSchema } from "./schemas";
+import { folderSchema, deckSchema, homeworkRecordSchema, answerRecordSchema, srsRecordSchema } from "./schemas";
 import { saveSettings, settingsStore } from "./settings";
 import { deckStore, replaceDecks } from "./storage";
 
@@ -25,12 +26,16 @@ const settingsSchema = z.object({
   haptics: z.boolean().optional(),
   locale: z.enum(["en", "cs"]).optional(),
   theme: z.enum(["light", "dark"]).optional(),
+  dailyGoal: z.number().int().min(1).max(500).optional(),
 });
 
 export const backupDataSchema = z.object({
   decks: z.array(deckSchema).max(2000),
   folders: z.array(folderSchema).max(500),
   homework: z.array(homeworkRecordSchema).max(2000),
+  /** Answer log and SRS schedule (IndexedDB). Optional for old backups. */
+  history: z.array(answerRecordSchema).max(20_000).optional(),
+  srs: z.array(srsRecordSchema).max(50_000).optional(),
 });
 
 export const backupSchema = z.object({
@@ -47,7 +52,8 @@ export type BackupFile = z.infer<typeof backupSchema>;
 /* Building and downloading                                            */
 /* ------------------------------------------------------------------ */
 
-export function buildBackup(): BackupFile {
+export async function buildBackup(): Promise<BackupFile> {
+  await ensureLearningHydrated();
   return {
     app: "freezmo",
     kind: "backup",
@@ -57,8 +63,11 @@ export function buildBackup(): BackupFile {
       decks: deckStore.getSnapshot(),
       folders: folderStore.getSnapshot(),
       homework: homeworkStore.getSnapshot(),
+      history: learningStore.getAnswersSnapshot(),
+      srs: learningStore.getSrsSnapshot(),
       settings: {
         haptics: settingsStore.getSnapshot().haptics,
+        dailyGoal: settingsStore.getSnapshot().dailyGoal,
         locale: (window.localStorage.getItem("freezmo.locale.v1") as "en" | "cs" | null) ?? undefined,
         theme: (window.localStorage.getItem("freezmo.theme") as "light" | "dark" | null) ?? undefined,
       },
@@ -66,8 +75,8 @@ export function buildBackup(): BackupFile {
   };
 }
 
-export function downloadBackup(): BackupFile {
-  const backup = buildBackup();
+export async function downloadBackup(): Promise<BackupFile> {
+  const backup = await buildBackup();
   const blob = new Blob([JSON.stringify(backup, null, 2)], { type: "application/json" });
   const url = URL.createObjectURL(blob);
   const anchor = document.createElement("a");
@@ -140,7 +149,10 @@ export function previewImport(backup: BackupFile): ImportPreview {
 }
 
 /** Applies a backup; the API key is never touched (it is not in the file). */
-export function applyImport(backup: BackupFile, mode: "merge" | "replace"): { decks: number; homework: number } {
+export async function applyImport(
+  backup: BackupFile,
+  mode: "merge" | "replace",
+): Promise<{ decks: number; homework: number }> {
   const data = backup.data;
   const decks =
     mode === "replace" ? data.decks : mergeById(deckStore.getSnapshot(), data.decks);
@@ -155,9 +167,27 @@ export function applyImport(backup: BackupFile, mode: "merge" | "replace"): { de
   replaceDecks(decks);
   replaceHomework(homework);
 
+  // Answer log + schedule: merge by id (answers) and by key (schedules).
+  await ensureLearningHydrated();
+  const incomingAnswers = data.history ?? [];
+  const incomingSrs = data.srs ?? [];
+  if (incomingAnswers.length > 0 || incomingSrs.length > 0) {
+    const currentAnswers = learningStore.getAnswersSnapshot();
+    const currentSrs = learningStore.getSrsSnapshot();
+    const answers =
+      mode === "replace" ? incomingAnswers : mergeById(currentAnswers, incomingAnswers);
+    const srsByKey = new Map(currentSrs.map((entry) => [entry.key, entry]));
+    for (const entry of incomingSrs) {
+      const current = srsByKey.get(entry.key);
+      if (!current || current.lastAt < entry.lastAt) srsByKey.set(entry.key, entry);
+    }
+    await replaceLearning(answers, [...srsByKey.values()]);
+  }
+
   const settings = data.settings;
   if (settings) {
     if (typeof settings.haptics === "boolean") saveSettings({ haptics: settings.haptics });
+    if (typeof settings.dailyGoal === "number") saveSettings({ dailyGoal: settings.dailyGoal });
     if (settings.locale) setLocale(settings.locale);
     if (settings.theme) applyTheme(settings.theme);
   }