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.

backoffice/app/Support/ClienteCredenziali.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Support;
+
+use App\Models\Cliente;
+use Illuminate\Support\Str;
+
+class ClienteCredenziali
+{
+    /** Genera un nome utente tipo "user2130" (numero casuale, minimo 4 cifre) non ancora in uso. */
+    public static function generaUsername(): string
+    {
+        do {
+            $username = 'user'.random_int(1000, 999999);
+        } while (Cliente::where('username', $username)->exists());
+
+        return $username;
+    }
+
+    /** Genera una password casuale sicura (lettere maiuscole/minuscole, numeri, simboli). */
+    public static function generaPassword(): string
+    {
+        return Str::password(14);
+    }
+}