fix: resolve pre-existing lint warnings

- Remove unused siteSettings import in auth-precheck.test.ts
- Replace non-null assertions with proper null checks in furni-data-i18n.ts
- Replace non-null assertions with proper null checks in conversion-pool.ts

src/actions/auth-precheck.test.ts

@@ -2,7 +2,6 @@
 import { beforeEach, describe, expect, it, vi } from "vitest";
 import { clientIp, rateLimit } from "@/lib/rate-limit";
 import { captchaConfig, verifyCaptcha } from "@/lib/services/captcha";
-import { siteSettings } from "@/lib/services/site-settings";
 import { precheckLogin } from "./auth-precheck";
 
 const core = vi.hoisted(() => ({