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diff --git a/src/client/db/index.ts b/src/client/db/index.ts
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+import Dexie, { type EntityTable } from "dexie";
+
+/**
+ * Card states for FSRS algorithm
+ */
+export const CardState = {
+	New: 0,
+	Learning: 1,
+	Review: 2,
+	Relearning: 3,
+} as const;
+
+export type CardStateType = (typeof CardState)[keyof typeof CardState];
+
+/**
+ * Rating values for reviews
+ */
+export const Rating = {
+	Again: 1,
+	Hard: 2,
+	Good: 3,
+	Easy: 4,
+} as const;
+
+export type RatingType = (typeof Rating)[keyof typeof Rating];
+
+/**
+ * Local deck stored in IndexedDB
+ * Includes _synced flag for offline sync tracking
+ */
+export interface LocalDeck {
+	id: string;
+	userId: string;
+	name: string;
+	description: string | null;
+	newCardsPerDay: number;
+	createdAt: Date;
+	updatedAt: Date;
+	deletedAt: Date | null;
+	syncVersion: number;
+	_synced: boolean;
+}
+
+/**
+ * Local card stored in IndexedDB
+ * Includes _synced flag for offline sync tracking
+ */
+export interface LocalCard {
+	id: string;
+	deckId: string;
+	front: string;
+	back: string;
+
+	// FSRS fields
+	state: CardStateType;
+	due: Date;
+	stability: number;
+	difficulty: number;
+	elapsedDays: number;
+	scheduledDays: number;
+	reps: number;
+	lapses: number;
+	lastReview: Date | null;
+
+	createdAt: Date;
+	updatedAt: Date;
+	deletedAt: Date | null;
+	syncVersion: number;
+	_synced: boolean;
+}
+
+/**
+ * Local review log stored in IndexedDB
+ * Includes _synced flag for offline sync tracking
+ * ReviewLog is append-only (no conflicts possible)
+ */
+export interface LocalReviewLog {
+	id: string;
+	cardId: string;
+	userId: string;
+	rating: RatingType;
+	state: CardStateType;
+	scheduledDays: number;
+	elapsedDays: number;
+	reviewedAt: Date;
+	durationMs: number | null;
+	syncVersion: number;
+	_synced: boolean;
+}
+
+/**
+ * Kioku local database using Dexie (IndexedDB wrapper)
+ *
+ * This database stores decks, cards, and review logs locally for offline support.
+ * Each entity has a _synced flag to track whether it has been synchronized with the server.
+ */
+export class KiokuDatabase extends Dexie {
+	decks!: EntityTable<LocalDeck, "id">;
+	cards!: EntityTable<LocalCard, "id">;
+	reviewLogs!: EntityTable<LocalReviewLog, "id">;
+
+	constructor() {
+		super("kioku");
+
+		this.version(1).stores({
+			// Primary key is 'id', indexed fields follow
+			// userId: for filtering by user
+			// updatedAt: for sync ordering
+			// Note: _synced is not indexed (boolean fields can't be indexed in IndexedDB)
+			// Use .filter() to find unsynced items
+			decks: "id, userId, updatedAt",
+
+			// deckId: for filtering cards by deck
+			// due: for finding due cards
+			// state: for filtering by card state
+			cards: "id, deckId, updatedAt, due, state",
+
+			// cardId: for finding reviews for a card
+			// userId: for filtering by user
+			// reviewedAt: for ordering reviews
+			reviewLogs: "id, cardId, userId, reviewedAt",
+		});
+	}
+}
+
+/**
+ * Singleton instance of the Kioku database
+ */
+export const db = new KiokuDatabase();