Get real mailbox size from dovecot

- Add mail.GetQuota() using doveadm quota get
- Fix UserWithQuota response struct
- Fix frontend NaN display with Unknown fallback
- Update field names to camelCase (usedQuota)

frontend/src/routes/domains/[name]/users/+page.svelte

@@ -4,9 +4,8 @@
   interface User {
     id: number;
     email: string;
-    domain_id: number;
     quota: number;
-    used_quota: number;
+    usedQuota?: number;
   }
 
   let users = $state<User[]>([]);
@@ -85,7 +84,8 @@
     }
   }
 
-  function formatQuota(bytes: number): string {
+  function formatQuota(bytes: number | undefined | null): string {
+    if (bytes === undefined || bytes === null || isNaN(bytes)) return 'Unknown';
     if (bytes === 0) return 'Default';
     if (bytes < 1024) return bytes + ' B';
     if (bytes < 1024 * 1024) return (bytes / 1024).toFixed(1) + ' KB';
@@ -127,7 +127,7 @@
         <tr>
           <td>{user.email}</td>
           <td>{formatQuota(user.quota)}</td>
-          <td>{formatQuota(user.used_quota)}</td>
+          <td>{formatQuota(user.usedQuota)}</td>
           <td>
             <button class="danger" onclick={() => deleteUser(user.id)}>Delete</button>
           </td>