Java Singleplayer World Download/Import System

Minecraft Java Edition needs a native singleplayer world downloading and importing system to safeguard builds and progress from device switches, crashes, or deletions. Unlike Bedrock's simple .mcworld exports, Java requires manual zipping of ~/.minecraft/saves folders or third-party tools, frustrating students on shared school laptops or family PCs—especially modders investing hours in survival worlds and custom maps. A "World Manager" in the Singleplayer menu fixes this. Tap "Download World" next to any listed world (with name, last played, size) for one-click .mcjava zip export—including level.dat thumbnail, "Quick Export" (~50MB session) or "Full Backup" (all dimensions), and chunk-based progress bar. Import via button/tab supports drag-and-drop .mcjava/.zip/.mcworld, QR scans, or cloud links, with auto-fixes for versions/mods ("Fabric? Enable loader?") and previews before saves folder placement. .mcjava zips world + JSON metadata (seed/version/mods/thumbnail), adds optional 5GB Mojang cloud sync, handles corruption/family profiles, runs lag-free (2GB cap), and includes passwords. This boosts Java's usability like Bedrock's portability, rescuing 100-day bases during hardware changes or 26.1 snapshot tests—saving time for solo creators everywhere.