feat: Implement client authentication and pre-fill functionality

- Added AuthController for client login and token management.
- Introduced AuthGate for handling authentication state and login UI.
- Updated API to support fetching client data for pre-filling forms.
- Modified App.vue to pre-fill client information if logged in.
- Enhanced client model to include username and password fields.
- Created migration for adding username and password to clients table.
- Added utility for generating secure usernames and passwords.
- Updated frontend to handle client tokens for pre-filling checkout forms.
- Removed old CSS file and updated HTML references to new assets.
- Added StatsOverview widget for displaying client and quote statistics.
- Improved error handling and user feedback in the login process.

src/js/api_base.js

@@ -0,0 +1,7 @@
+// Base URL dell'API Laravel: impostata in build (import.meta.env.VITE_PREVENTIVI_API_BASE, da
+// .env/.env.production — vedi root del repo), sovrascrivibile a runtime con window.PREVENTIVI_API_BASE
+// senza dover rebuildare (es. per un hotfix). Nessuna delle due impostata → fallback dev locale.
+export const API_BASE =
+    (typeof window.PREVENTIVI_API_BASE === 'string' && window.PREVENTIVI_API_BASE) ||
+    import.meta.env.VITE_PREVENTIVI_API_BASE ||
+    'http://localhost:8000/api';