refactor: replace Google Fonts with FontSource packages and migrate to cn utility

- Replace Google Fonts CDN with self-hosted FontSource packages
- Add @fontsource-variable/inter for variable Inter font
- Add @fontsource/ibm-plex-mono with weight 400 for monospace font
- Create src/lib/utils.ts with shadcn's cn utility function
- Install tailwind-merge for intelligent Tailwind class merging
- Replace all clsx usages with cn utility across components
  - LookupInput.tsx: 7 replacements
  - ErrorCard.tsx: 1 replacement
  - Property.tsx: 2 replacements
- Remove Google Fonts URL import from globals.css

src/pages/_app.tsx

@@ -1,5 +1,8 @@
 import { type AppType } from "next/dist/shared/lib/utils";
 
+import "@fontsource-variable/inter";
+import "@fontsource/ibm-plex-mono/400.css";
+
 import "../styles/globals.css";
 
 const MyApp: AppType = ({ Component, pageProps }) => {