holder: cap total cells at 1000 to prevent runaway renders

User-triggered 120x120 = 14400 cells, which produces huge STL/long
renders. Total cells is the right metric (CSG cost scales with count,
not max axis), so cap by N = rows*cols (or rows*(rows+1)/2 for tria
style). 1000 covers any realistic pack (e.g. 20x50) while blocking
accidental misuse.

Backend:
- holder.py: MAX_CELLS env-tunable (default 1000); expected_cell_count
  and _check_cell_limit raise ValueError on exceed; both
  compute_cells() and render_stl() call it up-front.
- app.py: /api/holder/render now returns 400 on ValueError (not 500)
  so the frontend can distinguish bad input from server failure.
  /api/holder/params now publishes max_cells alongside the schema.

Frontend:
- holder-app.js: reads max_cells from the params endpoint; status
  shows "N cells / over limit (1000)" in red and disables the
  Render and "Design busbars" buttons when exceeded.
- holder.css: .topbar-status.over-limit style (red, bold).

static/holder.css

@@ -105,3 +105,9 @@
 #btn-render.dirty {
   box-shadow: 0 0 0 2px var(--accent, #4a9eff) inset;
 }
+
+/* Cell-count over the server limit — render is disabled, status flagged */
+.topbar-status.over-limit {
+  color: var(--danger, #d94a4a);
+  font-weight: 600;
+}