reinstall instances

[objz]

what would you like

Allow a managed modpack instance to reinstall its provider files or switch to another compatible provider version without deleting user data.

completed in #31

• a shared refresh engine accepts any explicit provider version
• the target pack is built and validated in a staging instance
• pack-owned files can be replaced while worlds, configs, options, overrides, and user-added files are preserved
• user-file collisions are reviewed as keep or replace choices
• activation uses a rollback-only backup and restores the old instance if the swap fails
• v opens the provider version selector for a downloaded modpack instance
• selecting the installed version reinstalls the pack
• selecting another compatible version changes the pack version
• update, change, and reinstall reuse the same review and refresh flow

#31 closes this issue when merged.