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/postcss.config.js

@@ -0,0 +1,6 @@
+export default {
+  plugins: {
+    tailwindcss: {},
+    autoprefixer: {},
+  },
+}