holder: explicit Render button + progress UI + longer render timeout

- Replace debounced auto-render-on-param-change with an explicit Render
  button. Param changes mark the button "dirty" (accent ring); user clicks
  Render to drive a render. A Cancel button (AbortController) appears
  while a render is in flight.
- Add indeterminate progress bar with elapsed-time counter in the status
  panel. Real OpenSCAD --progress streaming can come later.
- Bump OPENSCAD_TIMEOUT default 60s -> 300s and gunicorn --timeout
  120s -> 300s. The 60s cap was misclassified by the frontend as
  "OpenSCAD not installed" because the error string contained the word
  "openscad" -- which the JS matched too greedily.
- Frontend error classifier now distinguishes "binary not found",
  "timed out", and "geometry empty" cases and only shows the
  install-OpenSCAD hint for the real not-found case.

static/js/holder-app.js

@@ -1,8 +1,8 @@
 /* holder-app.js — controller for the Hex Holder Designer page.
  *
  * - Loads /api/holder/params for the form schema and builds inputs.
- * - On any param change, debounces 400 ms then POSTs /api/holder/render and
- *   pushes the returned STL ArrayBuffer into the Three.js viewer.
+ * - Changing params marks the Render button "dirty"; user must click Render
+ *   (or Cancel) to drive POST /api/holder/render → Three.js viewer.
  * - "Download STL" re-uses the last rendered blob (no double trip).
  * - "Design busbars →" computes cells via /api/holder/cells, creates a new
  *   busbar-designer project via /api/projects, redirects to /?p=<id>.
@@ -14,8 +14,10 @@ const state = {
   params: {},        // current values, name → value
   schema: [],        // [{name, label, kind, ...}]
   lastBlob: null,    // last rendered STL blob (for download)
-  renderTimer: null,
   rendering: false,
+  dirty: false,      // params changed since last successful render
+  abortCtrl: null,   // for cancelling in-flight render
+  elapsedTimer: null,
   rendererReady: false,
 };
 
@@ -129,31 +131,56 @@ function _onParamChange(e) {
   else v = el.value;
   state.params[k] = v;
   _updateStatus();
-  _scheduleRender();
+  _markDirty();
 }
 
 // ----- render orchestration -------------------------------------------------
 
-function _scheduleRender() {
-  clearTimeout(state.renderTimer);
-  state.renderTimer = setTimeout(_doRender, 400);
+function _markDirty() {
+  state.dirty = true;
+  $("btn-render").classList.add("dirty");
+}
+
+function _clearDirty() {
+  state.dirty = false;
+  $("btn-render").classList.remove("dirty");
+}
+
+function _showProgress(show) {
+  $("render-progress").hidden = !show;
+  $("btn-render").hidden = show;
+  $("btn-cancel").hidden = !show;
+}
+
+function _classifyError(msg) {
+  // Backend RuntimeError messages all carry "openscad" in them. Distinguish:
+  if (/not found on PATH/i.test(msg))           return { hint: "OpenSCAD binary not found on the server (apt install openscad)." };
+  if (/timed out/i.test(msg))                   return { hint: "Render timed out. Try fewer cells, smaller slot widths, or simpler geometry." };
+  if (/produced no STL|geometry empty/i.test(msg)) return { hint: "OpenSCAD produced an empty model — check parameter combinations (e.g. cell_top_overlap too large)." };
+  return { hint: "" };
 }
 
 async function _doRender() {
-  if (state.rendering) {
-    // Re-schedule once current finishes.
-    state.renderTimer = setTimeout(_doRender, 200);
-    return;
-  }
+  if (state.rendering) return;
   state.rendering = true;
+  state.abortCtrl = new AbortController();
   $("render-status").textContent = "rendering…";
   $("warning").textContent = "";
+  $("render-time").textContent = "";
+  _showProgress(true);
+
   const t0 = performance.now();
+  state.elapsedTimer = setInterval(() => {
+    const dt = ((performance.now() - t0) / 1000).toFixed(1);
+    $("render-elapsed").textContent = `${dt}s`;
+  }, 100);
+
   try {
     const res = await fetch("/api/holder/render", {
       method: "POST",
       headers: { "content-type": "application/json" },
       body: JSON.stringify({ params: state.params }),
+      signal: state.abortCtrl.signal,
     });
     if (!res.ok) {
       let msg = res.statusText;
@@ -165,15 +192,20 @@ async function _doRender() {
     window.HolderViewer.loadSTL(buf);
     const dt = ((performance.now() - t0) / 1000).toFixed(1);
     $("render-status").textContent = `✓ rendered in ${dt}s · ${(state.lastBlob.size/1024).toFixed(0)} kB`;
-    $("render-time").textContent = "";
+    _clearDirty();
   } catch (e) {
-    $("render-status").textContent = `✗ ${e.message}`;
-    if (/openscad/i.test(e.message)) {
-      $("warning").textContent =
-        "Make sure OpenSCAD is installed on the server (apt install openscad).";
+    if (e.name === "AbortError") {
+      $("render-status").textContent = "✗ cancelled";
+    } else {
+      $("render-status").textContent = `✗ ${e.message}`;
+      const { hint } = _classifyError(e.message);
+      if (hint) $("warning").textContent = hint;
     }
   } finally {
     state.rendering = false;
+    state.abortCtrl = null;
+    clearInterval(state.elapsedTimer);
+    _showProgress(false);
   }
 }
 
@@ -191,6 +223,14 @@ function _updateStatus() {
 
 // ----- buttons --------------------------------------------------------------
 
+$("btn-render").addEventListener("click", () => {
+  if (state.rendererReady) _doRender();
+});
+
+$("btn-cancel").addEventListener("click", () => {
+  if (state.abortCtrl) state.abortCtrl.abort();
+});
+
 $("btn-download-stl").addEventListener("click", () => {
   if (!state.lastBlob) { alert("Nothing rendered yet."); return; }
   const url = URL.createObjectURL(state.lastBlob);
@@ -273,7 +313,9 @@ async function init() {
   _whenViewerReady(() => {
     window.HolderViewer.init($("viewer3d"));
     state.rendererReady = true;
-    _doRender();   // initial render with defaults
+    // Auto-render once on load so the user sees something immediately.
+    // After that, rendering is explicit (Render button).
+    _doRender();
   });
 }