fix(web): capture save previews from the browser canvas

Capture the rendered canvas during the clean frame instead of requesting synchronous GPU buffer readback. Preserve embedded previews and allow saving when canvas readback is denied. Add a browser save and reopen regression check.

Fixes #1021

src/app/update/file.rs

@@ -1704,19 +1704,14 @@ pub(super) fn on_open_file(&mut self) -> Task<Message> {
         let Some(pending) = self.pending_web_thumbnail_save.take() else {
             return Task::none();
         };
-        iced::window::latest()
-            .then(|window| match window {
-                Some(window) => iced::window::screenshot(window).map(Some),
-                None => Task::done(None),
-            })
-            .map(move |screenshot| Message::WebSaveScreenshot {
-                tab_id: pending.tab_id,
-                filename: pending.filename.clone(),
-                ext: pending.ext.clone(),
-                version: pending.version,
-                bounds: Some(pending.bounds),
-                screenshot,
-            })
+        Task::done(Message::WebSaveScreenshot {
+            tab_id: pending.tab_id,
+            filename: pending.filename,
+            ext: pending.ext,
+            version: pending.version,
+            bounds: Some(pending.bounds),
+            screenshot: crate::sys::capture_canvas(),
+        })
     }
 
     #[cfg(not(target_arch = "wasm32"))]