fix(notifications): set bg-canvas directly on <li> to prevent white bleed

The <a> inside each row has transparent background by default — CSS
background-color does not inherit. Putting bg-canvas only on the <ul>
was not enough; browsers still painted items white. Setting bg-canvas
on the <li> itself ensures the canvas color is explicitly applied to
each row in both light and dark mode.

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">
+					<li class="relative bg-canvas">
 						<a
 							href="/documents/{n.documentId}"
 							role="row"