fix(sessions): keep the tab put when a resource save fails

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KcyBRxE8j3ujCYkum9KrC4

frontend/src/lib/components/ResourceEditorDrawer.svelte

@@ -57,7 +57,8 @@
 
 	let resourceEditor:
 		| {
-				save: () => void
+				/** False when the write failed; it toasts its own error. */
+				save: () => Promise<boolean>
 				localDraftDeployed: () => unknown
 				localDraftCurrent: () => unknown
 				discardLocalDraft: () => void
@@ -205,9 +206,12 @@
 			// Closed before the write is awaited, the way it always was: `save()` toasts its
 			// own failures and never rejects, so waiting would only add visible lag to every
 			// caller of this drawer. `onSaved` still fires after the write lands.
-			const saved = resourceEditor?.save()
+			const saving = resourceEditor?.save()
 			drawer?.closeDrawer()
-			await saved
+			// Everything below moves this host onto the path that was written, so it
+			// must not run for a write that failed — a rejected rename (a name
+			// collision, say) would point the tab at a path this save never created.
+			if (!(await saving)) return
 			// Rendered inline there is no drawer to close, so the mounted editor would
 			// otherwise keep the pre-save baseline. Follow a rename before remounting,
 			// or it comes back up on a path the save just moved the item off.

frontend/src/lib/components/sessions/PreviewTabHost.svelte

@@ -86,13 +86,9 @@
 
 	let frame: HTMLIFrameElement | undefined = $state()
 
-	// Bumped to remount a hosted entity editor, which is how it re-reads the item
-	// from the server: its own state is built at mount from the draft cell, so
-	// there is nothing to refresh in place once that cell has been dropped. The
-	// acting workspace joins it in the key for the same reason — these editors
-	// load and save against the workspace they were mounted with, so a session
-	// that rescopes (a staged fork materialising on first send) must not leave
-	// them bound to the previous one.
+	// A hosted entity editor builds its state at mount, from the draft cell and the
+	// workspace it was given, and can refresh neither in place — so both the bump
+	// below and a change of `workspaceId` key it, and remounting is how it re-reads.
 	let entityNonce = $state(0)
 
 	// Pages whose theme we mirror on live toggles. Regular apps are the only item