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 <noreply@anthropic.com>

assets/icons/attsync.svg

@@ -0,0 +1,9 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="1.5" stroke-linecap="round" stroke-linejoin="round">
+  <!-- Sync arrows -->
+  <path d="M4 12 A8 8 0 0 1 18 7"/>
+  <polyline points="16,4 18,7 15,8"/>
+  <path d="M20 12 A8 8 0 0 1 6 17"/>
+  <polyline points="8,20 6,17 9,16"/>
+  <!-- Tag dot in center -->
+  <circle cx="12" cy="12" r="2" fill="currentColor" stroke="none"/>
+</svg>