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/styles.css

@@ -61,6 +61,27 @@ body {
   align-items: center;
 }
 
+.topbar-nav {
+  display: flex;
+  gap: 4px;
+  padding: 0 12px;
+  border-left: 1px solid var(--border);
+}
+
+.topbar-nav a {
+  color: var(--muted);
+  text-decoration: none;
+  padding: 4px 10px;
+  font-size: 13px;
+  border-bottom: 2px solid transparent;
+}
+
+.topbar-nav a:hover { color: var(--text); }
+.topbar-nav a.active {
+  color: var(--text);
+  border-bottom-color: var(--accent);
+}
+
 .project-bar {
   display: flex;
   align-items: center;