Co-locate CadCommand impls with their ribbon toolbar modules

Each command implementation is now in the same file as its ribbon
tool definition. Groups moved to a dedicated groups/ toolbar folder.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/modules/home/groups/mod.rs

@@ -0,0 +1,2 @@
+pub mod group;
+pub mod ungroup;