chore: switch from @tailwindcss/postcss to @tailwindcss/vite (Phase 2)

- Replaced @tailwindcss/postcss with @tailwindcss/vite plugin
- Deleted postcss.config.js (no longer needed)
- Tailwind now runs as a native Vite plugin for faster HMR

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/package.json

@@ -21,6 +21,7 @@
     "@sentry/react": "^10.42.0",
     "@sentry/vite-plugin": "^5.1.1",
     "@stripe/stripe-js": "^8.7.0",
+    "@tailwindcss/vite": "^4.2.1",
     "@xyflow/react": "^12.10.0",
     "axios": "^1.13.4",
     "class-variance-authority": "^0.7.1",
@@ -43,7 +44,6 @@
   },
   "devDependencies": {
     "@eslint/js": "^9.39.1",
-    "@tailwindcss/postcss": "^4.2.1",
     "@testing-library/jest-dom": "^6.9.1",
     "@testing-library/react": "^16.3.2",
     "@testing-library/user-event": "^14.6.1",