Replace extraction backend with SevenZipJBinding + Zip4j JVM sidecar

- New JVM sidecar (resources/extractor-jvm/) using SevenZipJBinding for
  RAR/7z/TAR and Zip4j for ZIP multipart, matching JDownloader 2 stack
- Auto/JVM/Legacy backend modes via RD_EXTRACT_BACKEND env variable
- Fallback to legacy UnRAR/7z when JVM runtime unavailable
- Fix isJvmRuntimeMissingError false positives on valid extraction errors
- Cache JVM layout resolution to avoid repeated filesystem checks
- Route nested ZIP extraction through JVM backend consistently

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

README.md

@@ -62,7 +62,8 @@ Requirements:
 - Node.js `20+` (recommended `22+`)
 - npm
 - Windows `10/11` (for packaging and regular desktop use)
-- Optional: 7-Zip/UnRAR for specific archive formats
+- Java Runtime `8+` (for SevenZipJBinding sidecar backend)
+- Optional fallback: 7-Zip/UnRAR if you force legacy extraction mode
 
 ```bash
 npm install
@@ -122,7 +123,7 @@ The app stores runtime files in Electron's `userData` directory, including:
 ## Troubleshooting
 
 - Download does not start: verify token and selected provider in Settings.
-- Extraction fails: check archive passwords and extraction tool availability.
+- Extraction fails: check archive passwords, JVM runtime (`resources/extractor-jvm`), or force legacy mode with `RD_EXTRACT_BACKEND=legacy`.
 - Very slow downloads: check active speed limit and bandwidth schedules.
 - Unexpected interruptions: enable reconnect and fallback providers.