Add configurable downloads folder cleanup strategy

[Tosox]

Task Description

As a user, I need control over how downloaded mod archives are managed after installation so that I can balance disk space usage against keeping archives cached for reinstalls. Currently all archives are kept in the downloads folder indefinitely with no cleanup.

Acceptance Criteria

• A cleanup strategy option is exposed in the user config
• Supported strategies: keep all (current behavior), keep latest version only, delete all after installation
• When "keep latest only" is selected, old version archives for a mod are deleted after a newer one is installed
• Default strategy is "keep latest only"

Additional Information

Relevant code: AppPaths.downloadsDir and archive download step in InstallCoordinator.installEntries()