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).

static/holder.html

@@ -56,6 +56,32 @@
       <p class="hint" id="render-time"></p>
       <p class="hint" id="warning"></p>
     </section>
+
+    <section class="panel">
+      <h2>Fit check</h2>
+
+      <label class="fit-toggle">
+        <input type="checkbox" id="show-max-box" checked>
+        <span>Show max bounding box</span>
+      </label>
+      <p class="hint" id="max-box-dims">— render first</p>
+
+      <hr class="fit-sep">
+
+      <label class="fit-toggle">
+        <input type="checkbox" id="show-enclosure">
+        <span>Show enclosure</span>
+      </label>
+      <div class="enc-inputs">
+        <input id="enc-w" type="number" min="1" step="1" placeholder="W">
+        <span>×</span>
+        <input id="enc-d" type="number" min="1" step="1" placeholder="D">
+        <span>×</span>
+        <input id="enc-h" type="number" min="1" step="1" placeholder="H">
+        <span class="unit">mm</span>
+      </div>
+      <p class="hint" id="fit-verdict"></p>
+    </section>
   </aside>
 
   <section class="right">