fix: extract Classic server zip after download completes

[bouclem]

Fixes #221

Problem

When setting up a Classic server version, DownloadData's constructor calls FileUtil.extractByExtension(...) on minecraft_server.zip immediately after queueDownload(...). But queueDownload only adds the entry to a queue - the actual download happens later, when performDownload(...) is called from TaskSetup / TaskUpdateLibraries.

So the extraction runs against a file that does not exist yet and throws NoSuchFileException. The exception is swallowed by printStackTrace, the constructor finishes, the zip later gets downloaded, but nothing ever extracts it. The user has to run cleanup and re-setup to get a working server.

Fix

• Stash the zip path and target jar path on the instance when a .zip server artifact is detected.
• Move the extractByExtension + Files.move step to run at the end of performDownload(...), after the zip has actually been written to disk.
• Drop the silent printStackTrace; the IOException now propagates like every other error in performDownload.

Behavior for non-Classic versions (where serverArtifact.url does not end with .zip) is unchanged.

Testing

• gradlew compileJava - clean build, no new warnings.