Exclude frontend from main TypeScript build

Frontend TypeScript files are built separately with esbuild and their own tsconfig.json that includes DOM types. This prevents DOM type errors in the main backend build.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tsconfig.json

@@ -28,6 +28,7 @@
     "node_modules",
     "dist",
     "public",
-    "scripts"
+    "scripts",
+    "src/frontend"
   ]
 }