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/xclip.svg

@@ -0,0 +1,10 @@
+<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">
+  <!-- Document with clip boundary -->
+  <rect x="4" y="3" width="12" height="16" rx="1"/>
+  <!-- Clip boundary (dashed polygon) -->
+  <polygon points="6,6 14,6 14,15 6,15" stroke-dasharray="3 2"/>
+  <!-- Scissors symbol -->
+  <line x1="16" y1="16" x2="21" y2="21"/>
+  <circle cx="18" cy="14" r="2"/>
+  <circle cx="20" cy="19" r="2"/>
+</svg>