Replace hanging STL dialog with local file picker

SUB Control/README.md

@@ -14,10 +14,10 @@ server, open a browser tab, or require an internet connection.
 The WebView is frameless and uses the compact title bar drawn by the interface.
 The vehicle rail includes a local STL viewer: choose **Load STL**, then drag to
 orbit, use the mouse wheel to zoom, or switch between shaded and edge-focused
-rendering. The chosen file is remembered in `model_settings.json`; only its
-local path is stored. STEP/STP CAD solids must first be tessellated/exported as
-STL because the embedded renderer intentionally does not include a full CAD
-kernel.
+rendering. A cached STL copy is stored locally under the ignored `runtime/`
+directory so the model can be restored on the next launch.
+STEP/STP CAD solids must first be tessellated/exported as STL because the
+embedded renderer intentionally does not include a full CAD kernel.
 
 ## Safety first