feat(wasm): web Save/Save As go through the dialog with a direct download

On the web, Save and Save As now open the in-app Save dialog (filename +
format) like the desktop, and the unsaved-changes prompt's Save routes
through it too — instead of an immediate rfd download. Confirm serializes
to bytes and downloads directly via a Blob + hidden `<a download>` click
(a user gesture), so there is no intermediate "click to download" link.

Replaces the rfd-based download_web / WebSaveResult round-trip with a
synchronous sys::download_bytes. Native save is unchanged.

Native + web build; 75 tests pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/sys.rs

@@ -32,3 +32,30 @@ pub fn handle_path(h: &rfd::FileHandle) -> std::path::PathBuf {
 pub fn handle_path(h: &rfd::FileHandle) -> std::path::PathBuf {
     std::path::PathBuf::from(h.file_name())
 }
+
+/// Trigger a browser download of `bytes` as `name`. Builds a Blob, points a
+/// hidden `<a download>` at it and clicks it programmatically — because this
+/// runs inside the Save button's click (a user gesture), the file downloads
+/// immediately with no extra "click to download" link. Web only.
+#[cfg(target_arch = "wasm32")]
+pub fn download_bytes(name: &str, bytes: &[u8]) {
+    use wasm_bindgen::JsCast;
+    let Some(window) = web_sys::window() else { return };
+    let Some(document) = window.document() else { return };
+    let array = js_sys::Uint8Array::from(bytes);
+    let parts = js_sys::Array::new();
+    parts.push(&array.buffer());
+    let Ok(blob) = web_sys::Blob::new_with_u8_array_sequence(&parts) else {
+        return;
+    };
+    let Ok(url) = web_sys::Url::create_object_url_with_blob(&blob) else {
+        return;
+    };
+    if let Ok(el) = document.create_element("a") {
+        let a: web_sys::HtmlAnchorElement = el.unchecked_into();
+        a.set_href(&url);
+        a.set_download(name);
+        a.click();
+    }
+    let _ = web_sys::Url::revoke_object_url(&url);
+}