fix(notifications): surface action failures as an error banner

When dismiss-notification or mark-all-read returns a failure the dropdown
now shows a localised error message above the list. Added
notification_error_generic key (de/en/es) as the fallback when the
action response carries no explicit error string.

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

frontend/messages/de.json

@@ -522,6 +522,7 @@
 	"notification_filter_unread": "Ungelesen",
 	"notification_filter_mention": "Erwähnung",
 	"notification_filter_reply": "Antwort",
+	"notification_error_generic": "Aktion fehlgeschlagen. Bitte versuche es erneut.",
 	"notification_mark_all_read_aria": "Alle Benachrichtigungen als gelesen markieren",
 	"notification_load_more": "Ältere laden",
 	"notification_empty_history": "Keine Benachrichtigungen",

frontend/messages/en.json

@@ -522,6 +522,7 @@
 	"notification_filter_unread": "Unread",
 	"notification_filter_mention": "Mention",
 	"notification_filter_reply": "Reply",
+	"notification_error_generic": "Action failed. Please try again.",
 	"notification_mark_all_read_aria": "Mark all notifications as read",
 	"notification_load_more": "Load older",
 	"notification_empty_history": "No notifications",

frontend/messages/es.json

@@ -522,6 +522,7 @@
 	"notification_filter_unread": "No leídas",
 	"notification_filter_mention": "Mención",
 	"notification_filter_reply": "Respuesta",
+	"notification_error_generic": "La acción ha fallado. Por favor, inténtalo de nuevo.",
 	"notification_mark_all_read_aria": "Marcar todas las notificaciones como leídas",
 	"notification_load_more": "Cargar anteriores",
 	"notification_empty_history": "Sin notificaciones",

frontend/src/lib/notification/NotificationDropdown.svelte

@@ -15,6 +15,8 @@ type Props = {
 
 let { notifications, optimisticMarkRead, optimisticMarkAllRead, onClose }: Props = $props();
 
+let errorMessage = $state<string | null>(null);
+
 function handleViewAll() {
 	onClose(); // close first — avoids stale dropdown during navigation transition
 	goto('/aktivitaeten');
@@ -38,7 +40,10 @@ function handleViewAll() {
 				method="POST"
 				use:enhance={() => {
 					optimisticMarkAllRead();
-					return async ({ update }) => {
+					return async ({ result, update }) => {
+						if (result.type === 'failure') {
+							errorMessage = (result.data as { error?: string } | undefined)?.error ?? m.notification_error_generic();
+						}
 						await update({ reset: false, invalidateAll: false });
 					};
 				}}
@@ -53,6 +58,11 @@ function handleViewAll() {
 		{/if}
 	</div>
 
+	<!-- Error banner (shown when a dismiss or mark-all action fails) -->
+	{#if errorMessage}
+		<p class="px-4 py-2 text-sm text-red-600">{errorMessage}</p>
+	{/if}
+
 	<!-- Notification list -->
 	{#if notifications.length === 0}
 		<!-- Empty state -->
@@ -85,7 +95,9 @@ function handleViewAll() {
 						use:enhance={() => {
 							optimisticMarkRead(notification.id);
 							return async ({ result, update }) => {
-								if (result.type !== 'failure') {
+								if (result.type === 'failure') {
+									errorMessage = (result.data as { error?: string } | undefined)?.error ?? m.notification_error_generic();
+								} else {
 									onClose();
 									goto(
 										buildCommentHref(