[FEATURE] Add a Ground Level setting to the GUI

Is your feature request related to a problem? Please describe. ground_level (the Y level generation is anchored to) is already a real setting on the CLI (--ground-level, defaults to -62) and it flows all the way through the backend, but there's no way to change it from the GUI — main.js hardcodes -62 regardless.

This isn't just cosmetic: this project already accepts that the world's height range isn't always vanilla's -64..319 (there's a bundled datapack that extends it to Y -2032..2031 on newer Java versions via --disable-height-limit). The same applies to mods that change the global build-height limits for all worlds — if someone's world has a different floor than vanilla's default, being stuck at a hardcoded -62 in the GUI can put the generated terrain in the wrong place relative to that world's actual bottom. CLI users can already work around this with --ground-level; GUI users can't.

Describe the solution you'd like Bring back the Ground Level input in the Settings panel and hook it back up to the value the app already sends to the backend, instead of the hardcoded -62. The old input and its translations (label + placeholder, still present in every locale file, just unused) are basically still there — it just needs to be un-commented/re-wired and added to the settings persistence list (settings-store.js) like the other numeric settings, so it saves/reverts/resets the same way. No Rust changes needed, the backend already accepts and forwards this value.

Additional context Not something that needs inventing from scratch — it's reverting a deliberate removal (commit 4fb4679, "Deprecate ground level input") that predates a lot of the current elevation/terrain work. Since it's already a first-class CLI option, restoring it keeps the GUI and CLI in sync instead of adding something new and isolated. If there was a reason for the original removal I'm not seeing, happy to hear it.

Happy to open a PR for this if you're on board.