chore: Revert to stable versions for Tailwind ecosystem

- tailwindcss: 4.2.0 (beta) -> 3.4.19 (stable)
- tailwind-merge: 3.5.0 (beta) -> 2.6.1 (stable)

Tailwind v4 is still in beta and requires breaking config changes.
Sticking with v3 for stability.

frontend/package.json

@@ -22,14 +22,14 @@
     "@radix-ui/react-toast": "^1.2.15",
     "class-variance-authority": "^0.7.1",
     "clsx": "^2.1.1",
-    "tailwind-merge": "^3.5.0"
+    "tailwind-merge": "^2.6.1"
   },
   "devDependencies": {
     "@types/node": "^22.12.0",
     "@types/react": "^19.1.0",
     "@types/react-dom": "^19.1.1",
     "typescript": "^5.9.3",
-    "tailwindcss": "^4.2.0",
+    "tailwindcss": "^3.4.19",
     "postcss": "^8.4.50",
     "autoprefixer": "^10.4.24"
   }