Feat/stairs fence update (#226)

* feat: railing on the straight stairs and new fence

* feat: added spiral and curved stairs with bug fix for fence

* feat:fence are linked to each other ... so moving one move the other sharing the same coordinate

* fix: update stair railing logic to include front-side attachments for terminal landings

* Integrate fence rendering into the fence system

* fix: pass nodeId instead of undefined node to WallTreeNode and FenceTreeNode

TreeNode was passing `node` (undefined variable) instead of `nodeId` to
WallTreeNode and FenceTreeNode, causing a runtime ReferenceError.
Updated FenceTreeNode to accept nodeId and look up the node from the
scene store internally, consistent with all other tree node components.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* feat: update fence icon with new isometric design

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Aymeric Rabot <aymeric@pascal.app>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

packages/viewer/src/components/viewer/index.tsx

@@ -3,6 +3,7 @@
 import {
   CeilingSystem,
   DoorSystem,
+  FenceSystem,
   ItemSystem,
   RoofSystem,
   SlabSystem,
@@ -138,6 +139,7 @@ const Viewer: React.FC<ViewerProps> = ({
       {/* Core systems */}
       <CeilingSystem />
       <DoorSystem />
+      <FenceSystem />
       <ItemSystem />
       <RoofSystem />
       <SlabSystem />