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).

app.py

@@ -56,7 +56,11 @@ def health():
 @app.get("/api/holder/params")
 def holder_params():
     """Schema for the parameter form + their defaults."""
-    return jsonify({"params": holder.schema_dict(), "defaults": holder.default_params()})
+    return jsonify({
+        "params":    holder.schema_dict(),
+        "defaults":  holder.default_params(),
+        "max_cells": holder.MAX_CELLS,
+    })
 
 
 @app.post("/api/holder/render")
@@ -65,6 +69,8 @@ def holder_render():
     body = request.get_json(silent=True) or {}
     try:
         data = holder.render_stl(body.get("params", {}))
+    except ValueError as e:
+        return jsonify({"error": str(e)}), 400
     except (FileNotFoundError, RuntimeError) as e:
         return jsonify({"error": str(e)}), 500
     return Response(