Match JDownloader 2 extraction behavior: normal I/O, -mt2 hybrid

- Remove setWindowsBackgroundIO entirely (JD2 uses normal I/O priority)
- Keep only CPU priority IDLE (os.setPriority)
- Hybrid threads fixed at -mt2 (matches JD2's ~16 MB/s controlled throughput)
- Final extraction uses full thread count (unchanged)

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.5.41",
+  "version": "1.5.42",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",