feat(auth): enhance validation rules for username and password

- Updated username validation to allow only lowercase letters, numbers, and underscores.
- Strengthened password requirements to include at least 8 characters, one uppercase letter, one lowercase letter, one number, and one special character.
- Adjusted frontend forms and backend DTOs to reflect new validation rules.

frontend/src/app/(auth)/login/page.tsx

@@ -30,7 +30,7 @@ const loginSchema = z.object({
 	email: z.string().email({ message: "Email invalide" }),
 	password: z
 		.string()
-		.min(6, { message: "Le mot de passe doit faire au moins 6 caractères" }),
+		.min(8, { message: "Le mot de passe doit faire au moins 8 caractères" }),
 });
 
 type LoginFormValues = z.infer<typeof loginSchema>;