feat(install): install system-wide under /usr/local with preflight checks

feat(install): install system-wide under /usr/local with preflight checks

- abort early with a clear error if mvn is missing or the available
  JDK is older than 21 (handles legacy 1.8 version scheme)
- hardcode the prefix to /usr/local so all users get the jar,
  launcher and desktop entry; drop CRT_BASIC_PREFIX and the per-user
  ~/.local default; require sudo and fail fast before building
- hide the .desktop entry from the app menu (NoDisplay) while keeping
  the text/x-basic "Open With" association
- document dependencies, install layout and window mouse-grab
  behavior in index.org; refresh crtbasic.png