fix(notifications): use bg-surface on <li> rows

bg-canvas matched the page background making rows invisible against it.
bg-surface gives each row the correct card/surface color (white in light,
dark panel in dark mode), matching what was always intended.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -205,7 +205,7 @@ function typeBadgeLabel(type: NotificationItem['type']): string {
 				class="divide-y divide-line rounded-sm border border-line bg-canvas shadow-sm"
 			>
 				{#each allNotifications as n (n.id)}
-					<li class="relative bg-canvas">
+					<li class="relative bg-surface">
 						<a
 							href="/documents/{n.documentId}"
 							role="row"