feat(#89): Add backup functionality for google drive and dropbox

src/lib/utils/oauthState.ts

@@ -0,0 +1,56 @@
+import { randomUUID } from 'node:crypto';
+import type { RequestEvent } from '@sveltejs/kit';
+
+export type OAuthStatePayload = {
+	state: string;
+	userId: string;
+	pokedexId?: string | null;
+	provider: string;
+	fileName?: string | null;
+	folderId?: string | null;
+	path?: string | null;
+	returnTo?: string | null;
+};
+
+const STATE_MAX_AGE = 10 * 60; // 10 minutes
+
+export function createOAuthState(): string {
+	return randomUUID();
+}
+
+function cookieName(provider: string): string {
+	return `oauth_state_${provider}`;
+}
+
+export function setOAuthStateCookie(
+	event: RequestEvent,
+	provider: string,
+	payload: OAuthStatePayload
+) {
+	event.cookies.set(cookieName(provider), JSON.stringify(payload), {
+		path: '/',
+		httpOnly: true,
+		sameSite: 'lax',
+		secure: event.url.protocol === 'https:',
+		maxAge: STATE_MAX_AGE
+	});
+}
+
+export function readOAuthStateCookie(
+	event: RequestEvent,
+	provider: string
+): OAuthStatePayload | null {
+	const raw = event.cookies.get(cookieName(provider));
+	if (!raw) return null;
+	try {
+		const parsed = JSON.parse(raw) as OAuthStatePayload;
+		if (!parsed?.state || !parsed?.userId) return null;
+		return parsed;
+	} catch {
+		return null;
+	}
+}
+
+export function clearOAuthStateCookie(event: RequestEvent, provider: string) {
+	event.cookies.delete(cookieName(provider), { path: '/' });
+}