fix(start): complete Start page workflow

Track Save As paths, persist the selected section, and size tabbed
panels from their actual available width.

Refs #443

src/app/update/mod.rs

@@ -224,6 +224,7 @@ impl OpenCADStudio {
 
             Message::StartSectionSelect(section) => {
                 self.start_section = section;
+                self.save_config();
                 Task::none()
             }