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.css

@@ -111,3 +111,35 @@
   color: var(--danger, #d94a4a);
   font-weight: 600;
 }
+
+/* Fit-check panel */
+.fit-toggle {
+  display: flex;
+  align-items: center;
+  gap: 6px;
+  font-size: 12px;
+  cursor: pointer;
+  user-select: none;
+}
+.fit-toggle input { margin: 0; }
+.fit-sep {
+  border: none;
+  border-top: 1px solid var(--border, #2a2f3a);
+  margin: 10px 0 8px;
+}
+.enc-inputs {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  margin: 6px 0;
+  font-size: 12px;
+}
+.enc-inputs input {
+  width: 56px;
+  padding: 3px 6px;
+  font-size: 12px;
+}
+.enc-inputs .unit { color: var(--muted); margin-left: 2px; }
+
+#fit-verdict.fit-ok    { color: #4ad97a; font-weight: 600; }
+#fit-verdict.fit-bad   { color: var(--danger, #d94a4a); font-weight: 600; }