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.html

@@ -28,7 +28,9 @@
   <span id="status" class="topbar-status">— cells</span>
 
   <div class="actions">
-    <button id="btn-download-stl" class="primary"  title="Download STL of the current configuration">Download STL</button>
+    <button id="btn-render" class="primary" title="Render STL with current parameters">Render</button>
+    <button id="btn-cancel" title="Cancel running render" hidden>Cancel</button>
+    <button id="btn-download-stl" title="Download STL of the current configuration">Download STL</button>
     <button id="btn-download-scad" title="Download the OpenSCAD source with current parameters baked in">Download .scad</button>
     <span class="sep"></span>
     <button id="btn-to-busbar" class="primary" title="Open Busbar Designer with these cell coordinates pre-loaded">Design busbars →</button>
@@ -47,6 +49,10 @@
     <section class="panel">
       <h2>Status</h2>
       <p class="hint" id="render-status">idle</p>
+      <div id="render-progress" class="render-progress" hidden>
+        <div class="render-progress-bar"><div class="render-progress-fill"></div></div>
+        <span id="render-elapsed" class="render-elapsed">0.0s</span>
+      </div>
       <p class="hint" id="render-time"></p>
       <p class="hint" id="warning"></p>
     </section>