feat(workspace): add real-time live cursor and presence tracking

- Support ephemeral cursor_moved and client_left events in docroom Hub
- Keep active in-memory cursor presence map per document room
- Synchronize current presence state to newly connected WebSocket clients
- Allow read-only viewers to share cursor positions without document mutation rights
- Throttle frontend cursor move emissions to 100ms with instant blur cleanup
- Render active collaborator presence avatar pills in document header
- Display user-colored block indicator and non-destructive caret overlay with name tag

frontend/src/features/workspace/WorkspaceView.tsx

@@ -864,6 +864,7 @@ export const WorkspaceView: React.FC<WorkspaceViewProps> = ({
           <DocumentEditor
             key={selectedDocId}
             docId={selectedDocId}
+            currentUser={currentUser}
             initialDoc={selectedDocObj}
             availableFolders={allFolderPaths}
             onDocMetadataChange={handleDocMetadataChange}