feat(view): show the 3D model on the 형상계획 and 철근배근 screens

The 3D viewer (views, canvas, part toggles) moves into a slot at the top of ②
and ③ while those screens are shown, and back to ① otherwise. Moving the
element keeps the WebGL context; the viewer is measured after its screen is
visible.

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

index.html

@@ -67,7 +67,7 @@
           </section>
         </aside>
 
-        <section class="panel viewer" aria-labelledby="viewer-title">
+        <section class="panel viewer" id="viewer-block" aria-labelledby="viewer-title">
           <div class="viewer-head">
             <h2 id="viewer-title">교각 3D 뷰</h2>
             <div id="views" class="views" role="group" aria-label="시점"></div>
@@ -107,6 +107,7 @@
       </main>
 
       <main class="grid plan" id="view-plan" hidden>
+        <div class="viewer-slot" id="slot-plan"></div>
         <aside class="panel piers" aria-labelledby="plan-pier-title">
           <p id="plan-pier-title" class="lbl">교각 목록</p>
           <div class="phead" aria-hidden="true"><span></span><span>교각</span><span>변경</span></div>
@@ -135,6 +136,7 @@
       </main>
 
       <main class="grid rebar" id="view-rebar" hidden>
+        <div class="viewer-slot" id="slot-rebar"></div>
         <aside class="panel piers" aria-labelledby="rebar-pier-title">
           <p id="rebar-pier-title" class="lbl">교각 목록</p>
           <div class="phead" aria-hidden="true"><span></span><span>교각</span><span>배근</span></div>