fix: show the right error when a model cannot be loaded for editing (#29694)

Opening the workspace model editor with an id that cannot be loaded redirected back to the model list and then fell through into the write-access check on the same empty model, so a plain not-found id produced "You do not have permission to edit this model". The message pointed people at their permissions for something that was never a permission problem.

The editor now stops after the redirect, so that message only appears when the model loaded and the user really cannot edit it.

Refs #29629

src/routes/(app)/workspace/models/edit/+page.svelte

@@ -24,6 +24,7 @@
 
 			if (!model) {
 				goto('/workspace/models');
+				return;
 			}
 
 			if (!model?.write_access) {