fix(quota): stop /me/quota leaking raw disk to guests

The per-user quota widget was shown to everyone and the /me/quota payload returned
free_disk_bytes (raw server free space) and active_uploaders to any authenticated
guest. Gate the widget to staff (host/admin) on the upload and account pages, and zero
the server-wide telemetry fields for non-staff in the handler. Guests still get their
own used/limit so enforcement stays transparent.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/routes/account/+page.svelte

@@ -431,8 +431,9 @@
 			</div>
 		</div>
 
-		<!-- Per-user quota widget -->
-		{#if $quotaStore.enabled && $quotaStore.limit_bytes != null}
+		<!-- Per-user quota widget — staff-only (host/admin); guests never see the
+			 server-derived storage figures. -->
+		{#if (role === 'host' || role === 'admin') && $quotaStore.enabled && $quotaStore.limit_bytes != null}
 			<div class="card p-5">
 				<h2
 					class="mb-2 text-xs font-semibold uppercase tracking-wide text-gray-500 dark:text-gray-400"