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/io/mod.rs

@@ -372,18 +372,6 @@ pub fn save_to_bytes(doc: &CadDocument, ext: &str) -> Result<Vec<u8>, String> {
     }
 }
 
-/// Web "save as": offer a download of `bytes` under `name` via the browser's
-/// save dialog. Returns the file name on success.
-#[cfg(target_arch = "wasm32")]
-pub async fn download_web(name: String, bytes: Vec<u8>) -> Result<String, String> {
-    let handle = rfd::AsyncFileDialog::new()
-        .set_file_name(&name)
-        .save_file()
-        .await
-        .ok_or_else(|| "Cancelled".to_string())?;
-    handle.write(&bytes).await.map_err(|e| e.to_string())?;
-    Ok(name)
-}
 
 // ── Post-load fixups ──────────────────────────────────────────────────────