fix(web): warn before losing unsaved changes

Closes #556

Cargo.toml

@@ -163,6 +163,7 @@ web-sys = { version = "0.3", features = [
     "KeyboardEventInit",
     "EventInit",
     "EventTarget",
+    "BeforeUnloadEvent",
     # Error banner (web_diag): forward wgpu/naga errors to the browser console
     # and inject a copyable DOM overlay (document.body → append_child).
     "console",

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
     }
 

src/sys.rs

@@ -17,6 +17,47 @@ pub fn open_url(url: &str) {
     }
 }
 
+#[cfg(target_arch = "wasm32")]
+thread_local! {
+    static BEFORE_UNLOAD_WARNING: std::cell::RefCell<
+        Option<wasm_bindgen::closure::Closure<dyn FnMut(web_sys::BeforeUnloadEvent)>>
+    > = const { std::cell::RefCell::new(None) };
+}
+
+/// Enable the browser's standard leave-page confirmation while drawings have
+/// unsaved changes. Browsers control the dialog text; preventing the event and
+/// setting `returnValue` are the portable signal that a warning is required.
+///
+/// The callback is installed only while needed so clean sessions remain
+/// eligible for the browser back/forward cache.
+#[cfg(target_arch = "wasm32")]
+pub fn set_unsaved_changes_warning(active: bool) {
+    use wasm_bindgen::JsCast;
+
+    let Some(window) = web_sys::window() else {
+        return;
+    };
+    BEFORE_UNLOAD_WARNING.with(|warning| {
+        let mut warning = warning.borrow_mut();
+        if active == warning.is_some() {
+            return;
+        }
+        if active {
+            let callback = wasm_bindgen::closure::Closure::new(
+                move |event: web_sys::BeforeUnloadEvent| {
+                    event.prevent_default();
+                    event.set_return_value("");
+                },
+            );
+            window.set_onbeforeunload(Some(callback.as_ref().unchecked_ref()));
+            *warning = Some(callback);
+        } else {
+            window.set_onbeforeunload(None);
+            warning.take();
+        }
+    });
+}
+
 /// Reveal a saved drawing in the native platform's file manager.
 #[cfg(not(target_arch = "wasm32"))]
 pub fn reveal_in_file_manager(path: &std::path::Path) -> Result<(), String> {