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/content_browser.svg

@@ -0,0 +1,13 @@
+<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">
+  <!-- Browser window -->
+  <rect x="2" y="3" width="20" height="18" rx="1"/>
+  <line x1="2" y1="7" x2="22" y2="7"/>
+  <!-- Nav buttons -->
+  <circle cx="5" cy="5" r="1" fill="currentColor" stroke="none"/>
+  <circle cx="9" cy="5" r="1" fill="currentColor" stroke="none"/>
+  <!-- Content grid -->
+  <rect x="4" y="10" width="6" height="5" rx="0.5"/>
+  <rect x="13" y="10" width="6" height="5" rx="0.5"/>
+  <rect x="4" y="17" width="6" height="3" rx="0.5"/>
+  <rect x="13" y="17" width="6" height="3" rx="0.5"/>
+</svg>