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/js/holder-app.js

@@ -19,6 +19,7 @@ const state = {
   abortCtrl: null,   // for cancelling in-flight render
   elapsedTimer: null,
   rendererReady: false,
+  maxCells: 1000,    // server-published cap; overridden by /api/holder/params
 };
 
 // ----- viewer init ----------------------------------------------------------
@@ -218,7 +219,16 @@ function _updateStatus() {
   let n;
   if (style === "tria") n = rows * (rows + 1) / 2;
   else n = rows * cols;
-  $("status").textContent = `${n} cells`;
+  const status = $("status");
+  const overLimit = n > state.maxCells;
+  status.textContent = overLimit
+    ? `${n} cells · over limit (${state.maxCells})`
+    : `${n} cells`;
+  status.classList.toggle("over-limit", overLimit);
+  // Block render when over the cap. Also blocks "Design busbars →"
+  // since it'd hit the same limit on the server.
+  $("btn-render").disabled = overLimit;
+  $("btn-to-busbar").disabled = overLimit;
 }
 
 // ----- buttons --------------------------------------------------------------
@@ -299,9 +309,10 @@ async function init() {
   try {
     const r = await fetch("/api/holder/params");
     if (!r.ok) throw new Error(r.statusText);
-    const { params: schema, defaults } = await r.json();
+    const { params: schema, defaults, max_cells } = await r.json();
     state.schema = schema;
     state.params = { ...defaults };
+    if (typeof max_cells === "number") state.maxCells = max_cells;
     _renderForm(schema, defaults);
     _updateStatus();
   } catch (e) {