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

.superpowers/brainstorm/269331-1778521434/state/server-stopped

@@ -0,0 +1 @@
+{"reason":"idle timeout","timestamp":1778524615786}