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/holder.css

@@ -65,3 +65,43 @@
   flex-basis: 100%;
   margin-top: -2px;
 }
+
+/* Render progress (indeterminate bar + elapsed-time counter) */
+.render-progress {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  margin: 6px 0;
+}
+.render-progress-bar {
+  flex: 1;
+  height: 6px;
+  border-radius: 3px;
+  background: var(--border, #2a2f3a);
+  overflow: hidden;
+  position: relative;
+}
+.render-progress-fill {
+  position: absolute;
+  inset: 0;
+  width: 35%;
+  border-radius: 3px;
+  background: linear-gradient(90deg, transparent, var(--accent, #4a9eff), transparent);
+  animation: render-progress-slide 1.2s linear infinite;
+}
+@keyframes render-progress-slide {
+  0%   { left: -35%; }
+  100% { left: 100%; }
+}
+.render-elapsed {
+  font-size: 11px;
+  color: var(--muted);
+  font-variant-numeric: tabular-nums;
+  min-width: 42px;
+  text-align: right;
+}
+
+/* Dirty indicator on Render button when params changed since last render */
+#btn-render.dirty {
+  box-shadow: 0 0 0 2px var(--accent, #4a9eff) inset;
+}