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.

holder.py

@@ -28,7 +28,7 @@ from typing import Any
 APP_DIR = Path(__file__).resolve().parent
 SCAD_FILE = APP_DIR / "scad" / "hex_cell.scad"
 OPENSCAD_BIN = os.environ.get("OPENSCAD_BIN", "openscad")
-RENDER_TIMEOUT = int(os.environ.get("OPENSCAD_TIMEOUT", "60"))
+RENDER_TIMEOUT = int(os.environ.get("OPENSCAD_TIMEOUT", "300"))
 
 
 def openscad_available() -> bool: