fix(web): warn before losing unsaved changes

Closes #556

src/app/mod.rs

@@ -3178,6 +3178,8 @@ pub fn run() -> iced::Result {
 
 impl Drop for OpenCADStudio {
     fn drop(&mut self) {
+        #[cfg(target_arch = "wasm32")]
+        crate::sys::set_unsaved_changes_warning(false);
         // Kill plugin runner processes as soon as the application state is
         // dropped, instead of waiting for the thread-local manager destructor.
         // This makes host shutdown deterministic and fast on every exit path.

src/app/update/mod.rs

@@ -217,6 +217,8 @@ impl OpenCADStudio {
                 );
             }
         }
+        #[cfg(target_arch = "wasm32")]
+        crate::sys::set_unsaved_changes_warning(self.tabs.iter().any(|tab| tab.dirty));
         task
     }