Add "Downloaded" timestamp column to completed downloads list

Display the completion time for each download in the done list.
The backend already stores a nanosecond timestamp on DownloadInfo;                                     this wires it up to the frontend using Angular's DatePipe.

ui/src/app/interfaces/download.ts

@@ -20,6 +20,7 @@ export interface Download {
   eta: number;
   filename: string;
   checked: boolean;
+  timestamp?: number;
   size?: number;
   error?: string;
   deleting?: boolean;