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/holder.css

@@ -0,0 +1,67 @@
+/* Holder-designer specific styles; complements styles.css. */
+
+.holder-left { width: 340px; }
+
+.viewer3d {
+  width: 100%;
+  height: 100%;
+  background: #0a0d12;
+}
+
+.right { position: relative; }
+
+/* Parameter form */
+.param-form {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+}
+
+.param-group {
+  border: 1px solid var(--border);
+  border-radius: 4px;
+  background: var(--bg);
+  padding: 8px 10px;
+}
+
+.param-group-title {
+  font-size: 11px;
+  color: var(--muted);
+  text-transform: uppercase;
+  letter-spacing: 0.05em;
+  margin: 0 0 6px;
+  font-weight: 600;
+}
+
+.param-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-bottom: 6px;
+}
+
+.param-row label {
+  flex: 1;
+  font-size: 12px;
+  color: var(--text);
+  text-transform: none;
+  letter-spacing: 0;
+  flex-direction: row;
+  align-items: center;
+}
+
+.param-row input[type=number],
+.param-row select {
+  width: 110px;
+  flex-shrink: 0;
+  font-size: 12px;
+  padding: 3px 6px;
+}
+
+.param-row .param-help {
+  font-size: 10px;
+  color: var(--muted);
+  flex-basis: 100%;
+  margin-top: -2px;
+}