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

Dockerfile

@@ -20,9 +20,11 @@ RUN pip install --user -r requirements.txt && \
 
 FROM python:3.12-slim AS runtime
 
-# OCP needs these at runtime.
+# OCP runtime libs + openscad (subprocess for /api/holder/render).
+# openscad headless renders STL from .scad with parameter overrides via -D.
 RUN apt-get update && apt-get install -y --no-install-recommends \
         libgl1 libglu1-mesa libxrender1 libxext6 libsm6 libgomp1 \
+        openscad xvfb \
     && rm -rf /var/lib/apt/lists/* \
     && useradd --create-home --shell /usr/sbin/nologin app