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

package.json

@@ -15,6 +15,8 @@
     "type-check": "tsc --noEmit"
   },
   "dependencies": {
+    "@fontsource-variable/inter": "^5.2.8",
+    "@fontsource/ibm-plex-mono": "^5.2.7",
     "@headlessui/react": "^2.2.9",
     "@heroicons/react": "^2.0.16",
     "@swc/helpers": "^0.5.11",
@@ -27,6 +29,7 @@
     "react-hook-form": "^7.42.1",
     "react-timeago": "^8.3.0",
     "sass": "^1.57.1",
+    "tailwind-merge": "^3.3.1",
     "true-myth": "^9.2.0",
     "usehooks-ts": "^3.1.1",
     "zod": "^4.1.12"