holder: fit-check — translucent max-box + manual enclosure overlay

The hex_cell.scad echoes its computed dimensions (pack_height_holder,
total_*_holder, box_total_*) to stderr. Capture those and surface them
in the viewer so the user can sanity-check whether the assembled
battery will fit in a target case.

Backend:
- holder.py: render_stl() now returns (stl_bytes, dims) where dims is
  a {name: float} dict parsed from openscad ECHO lines.
- app.py: /api/holder/render emits the dims dict as an
  X-Holder-Dimensions response header (JSON) with the matching
  Access-Control-Expose-Headers entry so fetch() can read it under
  any proxy / future CORS setup.

Viewer (holder-viewer.js):
- New setGhostBox(name, {w,d,h}, {visible,color}) and clearGhostBox(name)
  helpers. Each ghost is a Group of a translucent BoxGeometry mesh +
  matching EdgesGeometry wireframe, positioned to match how the STL
  mesh is placed (centred XY, bottom on Z=0).

UI (holder.html / holder.css):
- New "Fit check" panel under Status with two sections:
    • Show max bounding box (auto, from ECHO — defaults to box_total_*
      dims, falls back to total_*_holder + pack_height_holder).
    • Show enclosure (manual W × D × H inputs in mm).
- Verdict line under the enclosure inputs: "✓ fits" green or
  "✗ too small — battery won't fit" red.

Controller (holder-app.js):
- Reads X-Holder-Dimensions after each render, updates the max-box
  ghost in blue, prints the dimensions label.
- Watches enclosure inputs + toggles, drives the enclosure ghost
  (green if it fits, red if smaller than the max box on any axis).
- Fit comparison is orientation-independent in the XY plane (sorted
  W,D pair) but strict on Z (height).

app.py

@@ -10,6 +10,7 @@ Run:
 
 from __future__ import annotations
 
+import json
 import os
 import sys
 import traceback
@@ -68,7 +69,7 @@ def holder_render():
     """Render STL for the supplied parameter overrides."""
     body = request.get_json(silent=True) or {}
     try:
-        data = holder.render_stl(body.get("params", {}))
+        data, dims = holder.render_stl(body.get("params", {}))
     except ValueError as e:
         return jsonify({"error": str(e)}), 400
     except (FileNotFoundError, RuntimeError) as e:
@@ -76,7 +77,15 @@ def holder_render():
     return Response(
         data,
         mimetype="model/stl",
-        headers={"Content-Disposition": 'attachment; filename="hex_holder.stl"'},
+        headers={
+            "Content-Disposition": 'attachment; filename="hex_holder.stl"',
+            # ECHO values from hex_cell.scad (pack/box dimensions). Used by
+            # the viewer to draw a max-bounding-box ghost for fit-check.
+            "X-Holder-Dimensions": json.dumps(dims),
+            # Make the custom header visible to fetch() in any CORS/proxy
+            # setup (Access-Control-Expose-Headers; harmless on same-origin).
+            "Access-Control-Expose-Headers": "X-Holder-Dimensions",
+        },
     )