Add configurable parallel extraction count (1-8, default 2)

- New setting maxParallelExtract in AppSettings
- UI input in Entpacken tab: "Parallele Entpackungen"
- Replaces hardcoded maxConcurrent=2 in acquirePostProcessSlot

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

src/main/download-manager.ts

@@ -2997,7 +2997,7 @@ export class DownloadManager extends EventEmitter {
   }
 
   private async acquirePostProcessSlot(): Promise<void> {
-    const maxConcurrent = 2;
+    const maxConcurrent = this.settings.maxParallelExtract || 2;
     if (this.packagePostProcessActive < maxConcurrent) {
       this.packagePostProcessActive += 1;
       return;