Drop legacy JS heightmap tooling and dead resend command

- Remove dist_cfg/exts/*.js (compiler, import_sb_image, remove_empty_dirs,
  test_heightmap_import_export, transform_sb_image): superseded by the
  native heightmap/texture IO paths.
- Remove ResendCommand and its widget call site: the command was a
  never-implemented stub (only logged NOT IMPLEMENTED: RESEND), and the
  call site was unreachable behind `if not RELOAD_GADGETS` with the
  constant hardcoded true.
- Apply cargo fmt to project slice files (import order, line wrap).

Author: gajop