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

README.md

@@ -1,4 +1,15 @@
-# Busbar Designer
+# Busbar Designer + Hex Holder Designer
+
+Web "combine" for cylindrical-cell battery pack production. Two pages:
+
+| Page        | Purpose                                                                  |
+|-------------|---------------------------------------------------------------------------|
+| `/holder`   | Parametric **hex cell holder** designer — adjust pack size / cell type, see 3D, download STL for printing. |
+| `/`         | **Busbar** designer — import cell coordinates, group into busbars (parallel + series), export STEP/DXF/SVG for laser cutting. |
+
+The holder page has a **Design busbars →** button that auto-creates a busbar project pre-loaded with the exact cell coordinates from the configured holder.
+
+---
 
 Web tool for designing nickel/copper **busbars** over cylindrical-cell battery packs (21700, 18650, ...) built with hex-shaped cell holders.
 
@@ -11,9 +22,10 @@ Workflow:
 
 ## Quick start
 
-Requires **Python 3.10+**. On Windows / PowerShell:
+Requires **Python 3.10+** and **OpenSCAD** on the host (only needed for the `/holder` page; the busbar page works without it).
 
 ```powershell
+# Windows / PowerShell
 cd busbar-designer
 python -m venv .venv
 .\.venv\Scripts\Activate.ps1
@@ -21,7 +33,9 @@ pip install -r requirements.txt
 python app.py
 ```
 
-Then open <http://localhost:5000> in your browser.
+OpenSCAD install: Windows — `winget install OpenSCAD.OpenSCAD`; Linux — `apt install openscad`.
+
+Then open <http://localhost:5000> for the busbar designer, or <http://localhost:5000/holder> for the hex holder designer.
 
 > First-time install pulls **build123d** (~ 200 MB; bundles OpenCASCADE via OCP) and may take several minutes.