Fix browser file operations and persist visible save times

Preserve JSON Content-Type when custom idempotency headers are supplied. Expose the latest accepted folder operation timestamp so browser saves remain visible before an agent runs and after all files are deleted.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/shared/src/work-folders.ts

@@ -27,6 +27,7 @@ export interface WorkFolderListing {
   owner: WorkFolderOwner;
   files: WorkFile[];
   nextCursor: string | null;
+  lastSavedAt: string | null;
 }
 
 export interface WorkFolderSyncStatus {