Add hex holder designer page (/holder)

Server-side OpenSCAD renders STL from bundled hex_cell.scad with parameter
overrides via -D. Frontend is a Three.js viewer with auto-form generated
from /api/holder/params. 'Design busbars →' button posts the computed
cell coordinates to /api/projects and redirects to the busbar editor with
the holder cells pre-loaded.

  - holder.py:                openscad subprocess wrapper + compute_cells()
                              (Python mirror of get_hex_center_points_*)
  - scad/hex_cell.scad:       verbatim copy of Addy/Hex-Cell-Holder source
  - app.py:                   /holder route + /api/holder/{params,render,cells}
  - static/holder.html etc:   parameter form + Three.js STL viewer
  - Dockerfile / install.sh:  apt install openscad
  - static/index.html:        nav link Holder ↔ Busbars in topbar

static/index.html

@@ -10,6 +10,11 @@
 <header class="topbar">
   <h1>Busbar Designer</h1>
 
+  <nav class="topbar-nav">
+    <a href="/holder">Holder</a>
+    <a href="/" class="active">Busbars</a>
+  </nav>
+
   <div class="project-bar">
     <select id="project-select" title="Open project">
       <option value="">— select project —</option>