Add missing @tailwindcss/postcss dependency to frontend

Fix Docker build failure by adding @tailwindcss/postcss package
required by postcss.config.mjs

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

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

raggr-frontend/package.json

@@ -17,6 +17,7 @@
   "devDependencies": {
     "@rsbuild/core": "^1.5.6",
     "@rsbuild/plugin-react": "^1.4.0",
+    "@tailwindcss/postcss": "^4.0.0",
     "@types/react": "^19.1.13",
     "@types/react-dom": "^19.1.9",
     "typescript": "^5.9.2"