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

deploy/install.sh

@@ -43,7 +43,8 @@ apt-get update -qq
 apt-get install -y -qq \
     git ca-certificates curl \
     python3 python3-venv python3-pip \
-    libgl1 libglu1-mesa libxrender1 libxext6 libsm6 libgomp1
+    libgl1 libglu1-mesa libxrender1 libxext6 libsm6 libgomp1 \
+    openscad xvfb
 
 # ---- service user ----------------------------------------------------------
 if ! id "$SVC_USER" >/dev/null 2>&1; then