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/mview_block.svg

@@ -0,0 +1,14 @@
+<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">
+  <!-- Multi-view: 3 perspective projections of a block -->
+  <!-- Top view -->
+  <rect x="2" y="2" width="8" height="8" rx="0.5"/>
+  <!-- Front view -->
+  <rect x="2" y="14" width="8" height="8" rx="0.5"/>
+  <!-- Isometric block -->
+  <polygon points="14,8 20,5 20,15 14,18"/>
+  <polygon points="14,8 20,5 22,7 16,10"/>
+  <polygon points="16,10 22,7 22,17 16,20"/>
+  <line x1="14" y1="8" x2="16" y2="10"/>
+  <line x1="14" y1="18" x2="16" y2="20"/>
+  <line x1="20" y1="15" x2="22" y2="17"/>
+</svg>