feat: redesign Insert tab and fix all warnings

- Redesign Insert ribbon tab with 6 groups matching screenshot layout:
  Reference (Attach/Clip/Adjust large + Underlay Layers/Frames/Snap small),
  Point Cloud (Attach), Block (Multi-View Block/Insert large + Create/Edit/Base small),
  Attributes (Define/Edit large + Manage/Sync small),
  Import (Import/Land XML), Content (Content Browser/Design Center)
- Remove Primitives group and dead box_prim/clear/cylinder/sphere modules
- Fix all compiler warnings with #[allow(dead_code)] on unused-but-intentional items

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/modules/mod.rs

@@ -20,6 +20,7 @@ pub enum ModuleEvent {
     /// Open the OS file dialog.
     OpenFileDialog,
     /// Remove all loaded models from the scene.
+    #[allow(dead_code)]
     ClearModels,
     /// Toggle wireframe rendering.
     SetWireframe(bool),