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/shared/types.ts

@@ -65,6 +65,7 @@ export interface AppSettings {
   reconnectWaitSeconds: number;
   completedCleanupPolicy: FinishedCleanupPolicy;
   maxParallel: number;
+  maxParallelExtract: number;
   retryLimit: number;
   speedLimitEnabled: boolean;
   speedLimitKbps: number;