feat(01-03): add validation schemas and server actions

- Add Zod validation schemas for auth operations
- Add server actions for register, login, reset, update password
- Add clsx and tailwind-merge for class utilities
- Password validation: 8+ chars, 1 number, 1 uppercase
- Error messages in Italian per user requirement
- Specific error messages (not generic 'invalid credentials')

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

package.json

@@ -11,10 +11,12 @@
   "dependencies": {
     "@supabase/ssr": "^0.8.0",
     "@supabase/supabase-js": "^2.93.3",
+    "clsx": "^2.1.1",
     "next": "16.1.6",
     "react": "19.2.3",
     "react-dom": "19.2.3",
     "react-hook-form": "^7.71.1",
+    "tailwind-merge": "^3.4.0",
     "zod": "^4.3.6"
   },
   "devDependencies": {