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-lock.json

@@ -10,10 +10,12 @@
       "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": {
@@ -2692,6 +2694,15 @@
       "integrity": "sha512-IV3Ou0jSMzZrd3pZ48nLkT9DA7Ag1pnPzaiQhpW7c3RbcqqzvzzVu+L8gfqMp/8IM2MQtSiqaCxrrcfu8I8rMA==",
       "license": "MIT"
     },
+    "node_modules/clsx": {
+      "version": "2.1.1",
+      "resolved": "https://registry.npmjs.org/clsx/-/clsx-2.1.1.tgz",
+      "integrity": "sha512-eYm0QWBtUrBWZWG0d386OGAw16Z995PiOVo2B7bjWSbHedGl5e0ZWaq65kOGgUSNesEIDkB9ISbTg/JK9dhCZA==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=6"
+      }
+    },
     "node_modules/color-convert": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/color-convert/-/color-convert-2.0.1.tgz",
@@ -6176,6 +6187,16 @@
         "url": "https://github.com/sponsors/ljharb"
       }
     },
+    "node_modules/tailwind-merge": {
+      "version": "3.4.0",
+      "resolved": "https://registry.npmjs.org/tailwind-merge/-/tailwind-merge-3.4.0.tgz",
+      "integrity": "sha512-uSaO4gnW+b3Y2aWoWfFpX62vn2sR3skfhbjsEnaBI81WD1wBLlHZe5sWf0AqjksNdYTbGBEd0UasQMT3SNV15g==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/dcastil"
+      }
+    },
     "node_modules/tailwindcss": {
       "version": "4.1.18",
       "resolved": "https://registry.npmjs.org/tailwindcss/-/tailwindcss-4.1.18.tgz",