fix(notifications): move onClose/goto into enhance result callback

onClose() and goto() were firing before the server responded, making it
impossible for a fail() response to cancel navigation. Moved them inside
the result callback behind a result.type !== 'failure' guard.

Updated the $app/forms enhance mock to always invoke the returned async
callback with a configurable mockFormResult, and added three tests:
- success path calls onClose + goto with the correct deep-link URL
- failure path skips onClose and goto
- annotationId is appended to the URL when present

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

frontend/src/lib/notification/NotificationDropdown.svelte

@@ -84,15 +84,17 @@ function handleViewAll() {
 						class="contents"
 						use:enhance={() => {
 							optimisticMarkRead(notification.id);
-							onClose();
-							goto(
-								buildCommentHref(
-									notification.documentId,
-									notification.referenceId,
-									notification.annotationId
-								)
-							);
-							return async ({ update }) => {
+							return async ({ result, update }) => {
+								if (result.type !== 'failure') {
+									onClose();
+									goto(
+										buildCommentHref(
+											notification.documentId,
+											notification.referenceId,
+											notification.annotationId
+										)
+									);
+								}
 								await update({ reset: false, invalidateAll: false });
 							};
 						}}