feat(study): submit reviews offline via IndexedDB

Move FSRS scheduling to a shared module so the client can compute next
card state without contacting the server. StudyPage now writes the
updated card and review log straight to IndexedDB and lets the existing
sync engine push them on reconnect, instead of POSTing to
/api/decks/:deckId/study/:cardId. Online sessions still trigger a sync
immediately so server-side aggregates stay fresh; offline sessions
accumulate in pendingCountAtom until the next online tick.

The legacy study POST route is preserved for backwards compatibility.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 74 insertions, 0 deletions

diff --git a/src/shared/fsrs.ts b/src/shared/fsrs.ts
new file mode 100644
index 0000000..1cf60ba
--- /dev/null
+++ b/src/shared/fsrs.ts
@@ -0,0 +1,74 @@
+import {
+	type Card as FSRSCard,
+	type State as FSRSState,
+	fsrs,
+	type Grade,
+} from "ts-fsrs";
+
+const f = fsrs({ enable_fuzz: true });
+
+export interface ScheduleInput {
+	state: number;
+	due: Date;
+	stability: number;
+	difficulty: number;
+	elapsedDays: number;
+	scheduledDays: number;
+	reps: number;
+	lapses: number;
+	lastReview: Date | null;
+}
+
+export interface ScheduleResult {
+	state: number;
+	due: Date;
+	stability: number;
+	difficulty: number;
+	elapsedDays: number;
+	scheduledDays: number;
+	reps: number;
+	lapses: number;
+	lastReview: Date;
+	/** Days elapsed since the previous review (for ReviewLog). */
+	reviewElapsedDays: number;
+}
+
+export function computeNextSchedule(
+	card: ScheduleInput,
+	rating: 1 | 2 | 3 | 4,
+	now: Date,
+): ScheduleResult {
+	const fsrsCard: FSRSCard = {
+		due: card.due,
+		stability: card.stability,
+		difficulty: card.difficulty,
+		elapsed_days: card.elapsedDays,
+		scheduled_days: card.scheduledDays,
+		reps: card.reps,
+		lapses: card.lapses,
+		state: card.state as FSRSState,
+		last_review: card.lastReview ?? undefined,
+		learning_steps: 0,
+	};
+
+	const result = f.next(fsrsCard, now, rating as Grade);
+
+	const reviewElapsedDays = card.lastReview
+		? Math.round(
+				(now.getTime() - card.lastReview.getTime()) / (1000 * 60 * 60 * 24),
+			)
+		: 0;
+
+	return {
+		state: result.card.state,
+		due: result.card.due,
+		stability: result.card.stability,
+		difficulty: result.card.difficulty,
+		elapsedDays: result.card.elapsed_days,
+		scheduledDays: result.card.scheduled_days,
+		reps: result.card.reps,
+		lapses: result.card.lapses,
+		lastReview: now,
+		reviewElapsedDays,
+	};
+}