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/sys.rs

@@ -170,6 +170,48 @@ pub fn reveal_in_file_manager(path: &std::path::Path) -> Result<(), String> {
     }
 }
 
+/// Copy the rendered web canvas during the frame callback, before the browser
+/// clears its drawing buffer. Canvas readback avoids Iced's synchronous GPU map.
+#[cfg(target_arch = "wasm32")]
+pub fn capture_canvas() -> Option<iced::window::Screenshot> {
+    use wasm_bindgen::JsCast;
+
+    let window = web_sys::window()?;
+    let document = window.document()?;
+    let source = document
+        .query_selector("canvas")
+        .ok()??
+        .dyn_into::<web_sys::HtmlCanvasElement>()
+        .ok()?;
+    let (width, height) = (source.width(), source.height());
+    if width == 0 || height == 0 {
+        return None;
+    }
+    let copy = document
+        .create_element("canvas")
+        .ok()?
+        .dyn_into::<web_sys::HtmlCanvasElement>()
+        .ok()?;
+    copy.set_width(width);
+    copy.set_height(height);
+    let context = copy
+        .get_context("2d")
+        .ok()??
+        .dyn_into::<web_sys::CanvasRenderingContext2d>()
+        .ok()?;
+    context
+        .draw_image_with_html_canvas_element(&source, 0.0, 0.0)
+        .ok()?;
+    let pixels = context
+        .get_image_data(0.0, 0.0, width as f64, height as f64)
+        .ok()?;
+    Some(iced::window::Screenshot::new(
+        pixels.data().0,
+        iced::Size::new(width, height),
+        window.device_pixel_ratio() as f32,
+    ))
+}
+
 /// Web: read text from the system clipboard via the async Clipboard API.
 /// iced's own `clipboard::read` is a no-op on the web (the browser clipboard is
 /// async + permission-gated), so the editor paste paths use this instead. The