fix: resolve frontend code quality issues

- Create missing tsconfig.node.json for Vite config compilation
- Create postcss.config.js for Tailwind CSS processing
- Add test dependencies (vitest, testing-library, jsdom)
- Remove missing /vite.svg favicon reference from index.html

frontend/tsconfig.node.json

@@ -0,0 +1,10 @@
+{
+  "compilerOptions": {
+    "composite": true,
+    "skipLibCheck": true,
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "allowSyntheticDefaultImports": true
+  },
+  "include": ["vite.config.ts"]
+}