Log when each item download completes

Add "Download fertig: filename (size), pkg=name" log line when an item
finishes downloading, enabling precise timing analysis of when archive
parts become available for extraction.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/main/download-manager.ts

@@ -4842,6 +4842,7 @@ export class DownloadManager extends EventEmitter {
         item.updatedAt = nowMs();
         pkg.updatedAt = nowMs();
         this.recordRunOutcome(item.id, "completed");
+        logger.info(`Download fertig: ${item.fileName} (${humanSize(item.downloadedBytes)}), pkg=${pkg.name}`);
 
         if (this.session.running && !active.abortController.signal.aborted) {
           void this.runPackagePostProcessing(pkg.id).catch((err) => {