Detect dead links as permanent errors, fix last-episode extraction race

Dead link detection:
- Mega-Web: parse hoster error messages (hosterNotAvailable, etc.) from HTML
  and throw specific error instead of returning null
- MegaDebridClient: stop retrying on permanent hoster errors
- download-manager: isPermanentLinkError() immediately fails items with dead
  links instead of retrying forever

Extraction race condition:
- package_done cleanup policy checked if all items were "completed" (downloaded)
  but not if they were "extracted" — removing the package before the last
  episode could be extracted
- Both applyCompletedCleanupPolicy and applyPackageDoneCleanup now guard
  against premature removal when autoExtract is enabled

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

package.json

@@ -1,6 +1,6 @@
 {
   "name": "real-debrid-downloader",
-  "version": "1.4.89",
+  "version": "1.4.90",
   "description": "Real-Debrid Downloader Desktop (Electron + React + TypeScript)",
   "main": "build/main/main/main.js",
   "author": "Sucukdeluxe",