v1.2.1 — Internal refactor

v1.2.1 — Internal refactor

No user-facing changes. This release reorganizes the codebase from a
single ~1000-line main.go into focused internal packages
(config, roadmap, nbt, worlds, java, fabric, mojang, launcher, ui),
following the Single Responsibility Principle. This makes the codebase
easier to navigate, test, and extend — e.g. Forge/NeoForge support or
custom JVM arguments can now be added without touching unrelated code.

Functionality is identical to v1.2; binaries are rebuilt from the
refactored source and behave the same.