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>

Cargo.lock

@@ -26,6 +26,7 @@ dependencies = [
  "iced",
  "image",
  "inventory",
+ "js-sys",
  "lzma-sys",
  "ocs_plugin_api",
  "open",
@@ -39,6 +40,7 @@ dependencies = [
  "truck-shapeops",
  "ttf-parser",
  "ureq",
+ "wasm-bindgen",
  "web-sys",
  "windows-sys 0.61.2",
  "winresource",