feat(web): return saved drawings to an embedding page

The web control channel gains a save operation that serializes the
active drawing in the requested format and returns it as base64 in the
reply, clearing the dirty flag, the counterpart of opening from bytes.
A host page can also define window.openCadStudioSaveFile(name, bytes)
to receive every saved or exported file in place of a browser
download; while it is set no file picker opens.

Fixes #1704
Fixes #1705

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/sys.rs

@@ -597,10 +597,15 @@ pub fn download_bytes(name: &str, bytes: &[u8]) {
     let Some(window) = web_sys::window() else {
         return;
     };
+    let array = js_sys::Uint8Array::from(bytes);
+    // A host page that embeds the app takes the file instead of a download.
+    if let Some(hook) = save_hook() {
+        let _ = hook.call2(&window, &wasm_bindgen::JsValue::from_str(name), &array);
+        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 {
@@ -618,6 +623,18 @@ pub fn download_bytes(name: &str, bytes: &[u8]) {
     let _ = web_sys::Url::revoke_object_url(&url);
 }
 
+/// The `window.openCadStudioSaveFile(name, bytes)` function an embedding page
+/// registers to receive saved and exported files, if any.
+#[cfg(target_arch = "wasm32")]
+pub fn save_hook() -> Option<js_sys::Function> {
+    use wasm_bindgen::JsCast;
+    let window = web_sys::window()?;
+    js_sys::Reflect::get(window.as_ref(), &wasm_bindgen::JsValue::from_str("openCadStudioSaveFile"))
+        .ok()?
+        .dyn_into::<js_sys::Function>()
+        .ok()
+}
+
 /// Show a plot preview as an in-page overlay. The plotted PDF arrives already
 /// rasterised to PNG pages: inline `<img>` elements render in every browser,
 /// with no popup permission and no PDF viewer plugin — both of which embedded