refactor(chronik): replace callback props with form actions in ChronikFuerDichBox

Dismiss (X) button and mark-all-read button now submit forms to
/aktivitaeten?/dismiss-notification and /aktivitaeten?/mark-all-read respectively.
Props renamed onMarkRead/onMarkAllRead → optimisticMarkRead/optimisticMarkAllRead.

aktivitaeten/+page.svelte drops the now-deleted onMarkRead/onMarkAllRead wrapper functions
and passes notificationStore.optimisticMarkRead/optimisticMarkAllRead directly to the box.

Tests: $app/forms enhance mock added to both spec files so dismiss and mark-all assertions
work synchronously against form-submit events.

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

frontend/src/lib/activity/ChronikFuerDichBox.svelte

@@ -1,4 +1,5 @@
 <script lang="ts">
+import { enhance } from '$app/forms';
 import * as m from '$lib/paraglide/messages.js';
 import { relativeTime } from '$lib/shared/utils/time';
 import type { NotificationItem } from '$lib/notification/notifications.svelte';
@@ -6,11 +7,11 @@ import { buildCommentHref } from '$lib/shared/discussion/commentDeepLink';
 
 interface Props {
 	unread: NotificationItem[];
-	onMarkRead: (n: NotificationItem) => void;
-	onMarkAllRead: () => void;
+	optimisticMarkRead: (id: string) => void;
+	optimisticMarkAllRead: () => void;
 }
 
-const { unread, onMarkRead, onMarkAllRead }: Props = $props();
+const { unread, optimisticMarkRead, optimisticMarkAllRead }: Props = $props();
 
 function verb(type: NotificationItem['type'], actor: string): string {
 	return type === 'REPLY'
@@ -66,14 +67,24 @@ function href(n: NotificationItem): string {
 					{m.chronik_for_you_count({ count: unread.length })}
 				</span>
 			</div>
-			<button
-				type="button"
-				data-testid="chronik-mark-all-read"
-				onclick={onMarkAllRead}
-				class="font-sans text-xs font-medium text-ink-3 transition-colors hover:text-ink"
+			<form
+				action="/aktivitaeten?/mark-all-read"
+				method="POST"
+				use:enhance={() => {
+					optimisticMarkAllRead();
+					return async ({ update }) => {
+						await update({ reset: false, invalidateAll: false });
+					};
+				}}
 			>
-				{m.chronik_mark_all_read()}
-			</button>
+				<button
+					type="submit"
+					data-testid="chronik-mark-all-read"
+					class="font-sans text-xs font-medium text-ink-3 transition-colors hover:text-ink"
+				>
+					{m.chronik_mark_all_read()}
+				</button>
+			</form>
 		</div>
 
 		<ul role="list" class="flex flex-col gap-2">
@@ -89,7 +100,7 @@ function href(n: NotificationItem): string {
 							aria-hidden="true"
 							class="mt-0.5 inline-flex h-6 w-6 shrink-0 items-center justify-center rounded-full bg-accent-bg font-sans text-xs font-bold text-accent"
 						>
-							{n.type === 'MENTION' ? '@' : '\u21A9'}
+							{n.type === 'MENTION' ? '@' : '↩'}
 						</span>
 						<div class="min-w-0 flex-1">
 							<p class="font-sans text-sm leading-snug text-ink">
@@ -100,25 +111,36 @@ function href(n: NotificationItem): string {
 							</p>
 						</div>
 					</a>
-					<button
-						type="button"
-						data-testid="chronik-fuerdich-dismiss"
-						aria-label={m.chronik_mark_read_aria()}
-						onclick={() => onMarkRead(n)}
-						class="mt-0.5 shrink-0 rounded-sm p-1 text-ink-3 transition-colors hover:bg-muted hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
+					<form
+						action="/aktivitaeten?/dismiss-notification"
+						method="POST"
+						use:enhance={() => {
+							optimisticMarkRead(n.id);
+							return async ({ update }) => {
+								await update({ reset: false, invalidateAll: false });
+							};
+						}}
 					>
-						<svg
-							xmlns="http://www.w3.org/2000/svg"
-							class="h-4 w-4"
-							fill="none"
-							viewBox="0 0 24 24"
-							stroke="currentColor"
-							stroke-width="2"
-							aria-hidden="true"
+						<input type="hidden" name="notificationId" value={n.id} />
+						<button
+							type="submit"
+							data-testid="chronik-fuerdich-dismiss"
+							aria-label={m.chronik_mark_read_aria()}
+							class="mt-0.5 shrink-0 rounded-sm p-1 text-ink-3 transition-colors hover:bg-muted hover:text-ink focus-visible:ring-2 focus-visible:ring-focus-ring focus-visible:outline-none"
 						>
-							<path stroke-linecap="round" stroke-linejoin="round" d="M6 18L18 6M6 6l12 12" />
-						</svg>
-					</button>
+							<svg
+								xmlns="http://www.w3.org/2000/svg"
+								class="h-4 w-4"
+								fill="none"
+								viewBox="0 0 24 24"
+								stroke="currentColor"
+								stroke-width="2"
+								aria-hidden="true"
+							>
+								<path stroke-linecap="round" stroke-linejoin="round" d="M6 18L18 6M6 6l12 12" />
+							</svg>
+						</button>
+					</form>
 				</li>
 			{/each}
 		</ul>