fix(chronik): surface action failures in ChronikFuerDichBox with accessible error banner

Add $state errorMessage + role=alert banner to ChronikFuerDichBox. Both enhance callbacks
now inspect result.type and set the error message on 'failure' or 'error'; errorMessage
is cleared on each new submit attempt.

Upgrade both test files to the mockFormResult pattern (via vi.hoisted) so the result
callback is exercised. Add a failing-action test in each file that asserts role=alert
appears after a form submit with type='failure'.

Fix bare Function cast → explicit typed cast to satisfy @typescript-eslint/no-unsafe-function-type.

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

frontend/src/lib/activity/ChronikFuerDichBox.svelte

@@ -13,6 +13,8 @@ interface Props {
 
 const { unread, optimisticMarkRead, optimisticMarkAllRead }: Props = $props();
 
+let errorMessage: string | null = $state(null);
+
 function verb(type: NotificationItem['type'], actor: string): string {
 	return type === 'REPLY'
 		? m.notification_type_reply({ actor })
@@ -25,6 +27,9 @@ function href(n: NotificationItem): string {
 </script>
 
 <section class="rounded-sm border border-line bg-surface p-5">
+	{#if errorMessage}
+		<p role="alert" class="px-4 py-2 text-sm text-red-600">{errorMessage}</p>
+	{/if}
 	{#if unread.length === 0}
 		<div data-testid="chronik-inbox-zero" class="flex flex-col items-center gap-3 py-6 text-center">
 			<svg
@@ -71,8 +76,12 @@ function href(n: NotificationItem): string {
 				action="/aktivitaeten?/mark-all-read"
 				method="POST"
 				use:enhance={() => {
+					errorMessage = null;
 					optimisticMarkAllRead();
-					return async ({ update }) => {
+					return async ({ result, update }) => {
+						if (result.type === 'failure' || result.type === 'error') {
+							errorMessage = m.notification_error_generic();
+						}
 						await update({ reset: false, invalidateAll: false });
 					};
 				}}
@@ -115,8 +124,12 @@ function href(n: NotificationItem): string {
 						action="/aktivitaeten?/dismiss-notification"
 						method="POST"
 						use:enhance={() => {
+							errorMessage = null;
 							optimisticMarkRead(n.id);
-							return async ({ update }) => {
+							return async ({ result, update }) => {
+								if (result.type === 'failure' || result.type === 'error') {
+									errorMessage = m.notification_error_generic();
+								}
 								await update({ reset: false, invalidateAll: false });
 							};
 						}}