fix(notification-dropdown): handle error result type, add role=alert, fix update ordering

- Add role="alert" to error banner so screen-reader users hear failures
- Handle result.type === 'error' (network failure) alongside 'failure' in both enhance callbacks
- Clear errorMessage at the start of each submit so stale errors don't persist on retry
- On dismiss success: skip update() entirely since goto() navigates away from the page
- On dismiss failure: await update() then set error message
- On mark-all success: skip update() (optimistic state already applied)

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

frontend/src/lib/notification/NotificationDropdown.svelte

@@ -39,12 +39,13 @@ function handleViewAll() {
 				action="/aktivitaeten?/mark-all-read"
 				method="POST"
 				use:enhance={() => {
+					errorMessage = null;
 					optimisticMarkAllRead();
 					return async ({ result, update }) => {
-						if (result.type === 'failure') {
+						if (result.type === 'failure' || result.type === 'error') {
-							errorMessage = (result.data as { error?: string } | undefined)?.error ?? m.notification_error_generic();
+							errorMessage = (result as { data?: { error?: string } }).data?.error ?? m.notification_error_generic();
+							await update({ reset: false, invalidateAll: false });
 						}
-						await update({ reset: false, invalidateAll: false });
 					};
 				}}
 			>
@@ -60,7 +61,7 @@ function handleViewAll() {
 
 	<!-- 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>
+		<p role="alert" class="px-4 py-2 text-sm text-red-600">{errorMessage}</p>
 	{/if}
 
 	<!-- Notification list -->
@@ -93,11 +94,14 @@ function handleViewAll() {
 						method="POST"
 						class="contents"
 						use:enhance={() => {
+							errorMessage = null;
 							optimisticMarkRead(notification.id);
 							return async ({ result, update }) => {
-								if (result.type === 'failure') {
+								if (result.type === 'failure' || result.type === 'error') {
-									errorMessage = (result.data as { error?: string } | undefined)?.error ?? m.notification_error_generic();
+									errorMessage = (result as { data?: { error?: string } }).data?.error ?? m.notification_error_generic();
+									await update({ reset: false, invalidateAll: false });
 								} else {
+									// Navigate away — no need to update the store since we're leaving the page
 									onClose();
 									goto(
 										buildCommentHref(
@@ -107,7 +111,6 @@ function handleViewAll() {
 										)
 									);
 								}
-								await update({ reset: false, invalidateAll: false });
 							};
 						}}
 					>