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 <noreply@anthropic.com>

src/app/mod.rs

@@ -686,9 +686,6 @@ pub enum Message {
     FileOpened(Result<(String, PathBuf, CadDocument, crate::scene::DerivedCaches), String>),
     SaveFile,
     SaveAs,
-    /// Result of the web (browser download) save: file name or error.
-    #[cfg(target_arch = "wasm32")]
-    WebSaveResult(Result<String, String>),
     // ── Custom Save-As dialog ─────────────────────────────────────────────
     SaveDialogFormatChanged(String),
     SaveDialogFilenameChanged(String),