feat(wasm): save files from the browser

Wire up save for the web build:

- `io::save_to_bytes`: serialize a document to a byte buffer (DXF via
  write_to_vec, DWG via write_to_writer over a Cursor).
- `io::download_web`: browser save dialog → write bytes (triggers a
  download).
- Save and Save As on wasm both go through a new `web_save` helper:
  serialize the active tab and offer a download (no persistent path or
  in-app save dialog on the web). Format follows the tab name's
  extension, defaulting to DWG. Result reported via `WebSaveResult`.

Native build + 75 tests unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -678,6 +678,9 @@ 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),