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/styles/globals.css

@@ -1,5 +1,4 @@
 @import "tailwindcss";
-@import url("https://fonts.googleapis.com/css2?family=IBM+Plex+Mono:ital,wght@0,100;0,200;0,300;0,400;0,500;0,600;0,700;1,100;1,200;1,300;1,400;1,500;1,600;1,700&family=Inter:wght@100..900&display=swap");
 
 @theme {
   --font-sans: "Inter var", ui-sans-serif, system-ui, sans-serif, "Apple Color Emoji", "Segoe UI Emoji", "Segoe UI Symbol", "Noto Color Emoji";
@@ -7,8 +6,6 @@
   --color-zinc-850: #1D1D20;
 }
 
-@source "../**/*.{js,ts,jsx,tsx}";
-
 dd {
   margin: 0.5em 0 1em 2em;
 }