perf: code-split vendor chunks to eliminate 500kB bundle warning

Split the single 713kB JS bundle into separate chunks:
- react-vendor: React core (~192kB)
- markdown: react-markdown + remark/rehype + highlight.js (~476kB)
- icons: lucide-react (~6kB)
- ui: Radix UI components
- index: app code (~37kB)

Improves caching (vendor chunks change less often) and eliminates
the Vite chunk size warning.

vite.config.ts

@@ -4,4 +4,24 @@ import tailwindcss from '@tailwindcss/vite'
 
 export default defineConfig({
   plugins: [react(), tailwindcss()],
+  build: {
+    rollupOptions: {
+      output: {
+        manualChunks(id) {
+          if (id.includes('node_modules/react-dom') || id.includes('node_modules/react/')) {
+            return 'react-vendor'
+          }
+          if (id.includes('node_modules/react-markdown') || id.includes('node_modules/remark-') || id.includes('node_modules/rehype-') || id.includes('node_modules/unified') || id.includes('node_modules/mdast') || id.includes('node_modules/hast') || id.includes('node_modules/micromark') || id.includes('node_modules/highlight.js')) {
+            return 'markdown'
+          }
+          if (id.includes('node_modules/lucide-react')) {
+            return 'icons'
+          }
+          if (id.includes('node_modules/@radix-ui')) {
+            return 'ui'
+          }
+        },
+      },
+    },
+  },
 })