feat: add Modrinth integration (#255)

Add ability to search, install and manage mods from Modrinth: - Search mods filtered by Minecraft version and loader - One-click install from search results - Check for updates on installed Modrinth mods - Update mods to latest compatible version Creates a dedicated "Mods" tab in the version selector. Mods are downloaded to the custom_mods folder with metadata tracking. **Example:** Go to Mods tab, search "sodium", click install, and the mod will be ready for your next launch. **Safe:** Metadata stored in `.modrinth_meta.json` tracks installed mods. Updating a mod removes the old file before downloading the new one. Closes #255 ## Screenshots