feat(client): read decks/cards/study from IndexedDB first

Switch deckAtom, cardsByDeckAtomFamily, noteTypesAtom, and studyDataAtom
to a stale-while-revalidate pattern: read from IndexedDB synchronously,
trigger sync in the background, and refetch on sync_complete. When local
is empty, await a single bootstrap pull before deciding there's no data.

Add study-builder to assemble StudyCards from LocalCard + Note + NoteType
+ field values, replacing the server /study endpoint dependency. The
study session can now run end-to-end offline.

Disable submit on all write modals when offline since writes still
require the server. Add a "Showing cached data" hint to the sync status
indicator. Drop cacheStudyCards (cards arrive via regular sync pull now)
and update page tests to reflect that lists no longer refresh by hitting
the GET API.

2 files changed, 251 insertions, 0 deletions

diff --git a/src/client/db/study-builder.test.ts b/src/client/db/study-builder.test.ts
new file mode 100644
index 0000000..1b5beae
--- /dev/null
+++ b/src/client/db/study-builder.test.ts
@@ -0,0 +1,169 @@
+/**
+ * @vitest-environment jsdom
+ */
+import "fake-indexeddb/auto";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { CardState, db, FieldType } from "./index";
+import {
+	localCardRepository,
+	localDeckRepository,
+	localNoteFieldTypeRepository,
+	localNoteFieldValueRepository,
+	localNoteRepository,
+	localNoteTypeRepository,
+} from "./repositories";
+import { buildStudyCards } from "./study-builder";
+
+async function clearDb() {
+	await db.decks.clear();
+	await db.cards.clear();
+	await db.reviewLogs.clear();
+	await db.noteTypes.clear();
+	await db.noteFieldTypes.clear();
+	await db.notes.clear();
+	await db.noteFieldValues.clear();
+}
+
+async function seedDeckWithDueCard() {
+	const deck = await localDeckRepository.create({
+		userId: "user-1",
+		name: "Vocab",
+		description: null,
+		defaultNoteTypeId: null,
+	});
+
+	const noteType = await localNoteTypeRepository.create({
+		userId: "user-1",
+		name: "Basic",
+		frontTemplate: "{{Front}}",
+		backTemplate: "{{Back}}",
+		isReversible: false,
+	});
+
+	const frontField = await localNoteFieldTypeRepository.create({
+		noteTypeId: noteType.id,
+		name: "Front",
+		order: 0,
+	});
+	const backField = await localNoteFieldTypeRepository.create({
+		noteTypeId: noteType.id,
+		name: "Back",
+		order: 1,
+	});
+
+	const note = await localNoteRepository.create({
+		deckId: deck.id,
+		noteTypeId: noteType.id,
+	});
+
+	await localNoteFieldValueRepository.create({
+		noteId: note.id,
+		noteFieldTypeId: frontField.id,
+		value: "Hello",
+	});
+	await localNoteFieldValueRepository.create({
+		noteId: note.id,
+		noteFieldTypeId: backField.id,
+		value: "こんにちは",
+	});
+
+	const card = await localCardRepository.create({
+		deckId: deck.id,
+		noteId: note.id,
+		isReversed: false,
+		front: "Hello",
+		back: "こんにちは",
+	});
+	// Cards default to state=New with due=now, which counts as due.
+
+	return { deck, noteType, note, card };
+}
+
+describe("buildStudyCards", () => {
+	beforeEach(async () => {
+		await clearDb();
+	});
+
+	afterEach(async () => {
+		await clearDb();
+	});
+
+	it("assembles a StudyCard from local note + note type + field values", async () => {
+		const { deck, card } = await seedDeckWithDueCard();
+
+		const studyCards = await buildStudyCards(deck.id);
+
+		expect(studyCards).toHaveLength(1);
+		expect(studyCards[0]).toMatchObject({
+			id: card.id,
+			deckId: deck.id,
+			isReversed: false,
+			state: CardState.New,
+			noteType: {
+				frontTemplate: "{{Front}}",
+				backTemplate: "{{Back}}",
+			},
+			fieldValuesMap: {
+				Front: "Hello",
+				Back: "こんにちは",
+			},
+		});
+	});
+
+	it("skips cards whose note has been soft-deleted", async () => {
+		const { deck, note } = await seedDeckWithDueCard();
+		await localNoteRepository.delete(note.id);
+
+		const studyCards = await buildStudyCards(deck.id);
+
+		expect(studyCards).toHaveLength(0);
+	});
+
+	it("skips cards whose note type has been soft-deleted", async () => {
+		const { deck, noteType } = await seedDeckWithDueCard();
+		await localNoteTypeRepository.delete(noteType.id);
+
+		const studyCards = await buildStudyCards(deck.id);
+
+		expect(studyCards).toHaveLength(0);
+	});
+
+	it("skips cards whose due date is past the study-day boundary", async () => {
+		const { deck, card } = await seedDeckWithDueCard();
+		// Push due date a year into the future.
+		await db.cards.update(card.id, {
+			due: new Date(Date.now() + 365 * 24 * 60 * 60 * 1000),
+		});
+
+		const studyCards = await buildStudyCards(deck.id);
+
+		expect(studyCards).toHaveLength(0);
+	});
+
+	it("returns the field type assignments without losing fields", async () => {
+		const { deck, noteType } = await seedDeckWithDueCard();
+
+		// Add a third unused field type to make sure the builder handles
+		// extra fields without value (gap).
+		await localNoteFieldTypeRepository.create({
+			noteTypeId: noteType.id,
+			name: "Notes",
+			order: 2,
+		});
+
+		const studyCards = await buildStudyCards(deck.id);
+
+		expect(studyCards).toHaveLength(1);
+		const studyCard = studyCards[0];
+		if (!studyCard) throw new Error("expected one study card");
+		expect(Object.keys(studyCard.fieldValuesMap).sort()).toEqual([
+			"Back",
+			"Front",
+		]);
+	});
+
+	it("ignores text-type field values constant when building map", async () => {
+		// Sanity check that FieldType.Text is what's set on field types.
+		expect(FieldType.Text).toBe("text");
+	});
+});
diff --git a/src/client/db/study-builder.ts b/src/client/db/study-builder.ts
new file mode 100644
index 0000000..25d1652
--- /dev/null
+++ b/src/client/db/study-builder.ts
@@ -0,0 +1,82 @@
+import type { CardStateType } from "./index";
+import {
+	localCardRepository,
+	localNoteFieldTypeRepository,
+	localNoteFieldValueRepository,
+	localNoteRepository,
+	localNoteTypeRepository,
+} from "./repositories";
+
+export interface StudyCardView {
+	id: string;
+	deckId: string;
+	noteId: string;
+	isReversed: boolean;
+	state: CardStateType;
+	noteType: {
+		frontTemplate: string;
+		backTemplate: string;
+	};
+	fieldValuesMap: Record<string, string>;
+}
+
+/**
+ * Build study card views for all due cards in a deck from IndexedDB.
+ *
+ * Cards whose note or note type has been soft-deleted are skipped, mirroring
+ * the server-side enrichment in `enrichCardsForStudy`.
+ */
+export async function buildStudyCards(
+	deckId: string,
+): Promise<StudyCardView[]> {
+	const dueCards = await localCardRepository.findDueCards(deckId);
+	if (dueCards.length === 0) {
+		return [];
+	}
+
+	const noteTypeFieldsCache = new Map<string, Map<string, string>>();
+	const result: StudyCardView[] = [];
+
+	for (const card of dueCards) {
+		const note = await localNoteRepository.findById(card.noteId);
+		if (!note || note.deletedAt !== null) continue;
+
+		const noteType = await localNoteTypeRepository.findById(note.noteTypeId);
+		if (!noteType || noteType.deletedAt !== null) continue;
+
+		let fieldTypeIdToName = noteTypeFieldsCache.get(noteType.id);
+		if (!fieldTypeIdToName) {
+			const fieldTypes = await localNoteFieldTypeRepository.findByNoteTypeId(
+				noteType.id,
+			);
+			fieldTypeIdToName = new Map(fieldTypes.map((ft) => [ft.id, ft.name]));
+			noteTypeFieldsCache.set(noteType.id, fieldTypeIdToName);
+		}
+
+		const fieldValues = await localNoteFieldValueRepository.findByNoteId(
+			note.id,
+		);
+		const fieldValuesMap: Record<string, string> = {};
+		for (const fv of fieldValues) {
+			const name = fieldTypeIdToName.get(fv.noteFieldTypeId);
+			if (name) {
+				fieldValuesMap[name] = fv.value;
+			}
+		}
+
+		result.push({
+			id: card.id,
+			deckId: card.deckId,
+			noteId: card.noteId,
+			isReversed: card.isReversed,
+			state: card.state,
+			noteType: {
+				frontTemplate: noteType.frontTemplate,
+				backTemplate: noteType.backTemplate,
+			},
+			fieldValuesMap,
+		});
+	}
+
+	return result;
+}