fix(notification): replace view-all anchor with button to prevent iframe navigation

SvelteKit's capture-phase link interceptor fires before the component's
onclick handler, so e.preventDefault() was structurally too late to stop
iframe navigation in vitest-browser. Replacing the <a href> with a
<button type="button"> removes the href entirely — the interceptor never
fires — and the existing goto() mock in tests is sufficient.

Also splits the single view-all test into two focused it() blocks and
clears mocks in afterEach to prevent cross-test mock leakage.

Fixes #551

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

frontend/src/lib/notification/NotificationDropdown.svelte

@@ -13,8 +13,7 @@ type Props = {
 
 let { notifications, onMarkRead, onMarkAllRead, onClose }: Props = $props();
 
-function handleViewAll(e: MouseEvent) {
-	e.preventDefault();
+function handleViewAll() {
 	onClose();
 	goto('/aktivitaeten');
 }
@@ -134,12 +133,12 @@ function handleViewAll(e: MouseEvent) {
 	{/if}
 
 	<div class="border-t border-line px-4 py-2">
-		<a
-			href="/aktivitaeten"
+		<button
+			type="button"
 			onclick={handleViewAll}
 			class="text-xs font-medium text-ink-2 transition-colors hover:text-ink"
 		>
 			{m.chronik_view_all()}
-		</a>
+		</button>
 	</div>
 </div>