fix: Donate button (Start tab gate + web window.open)

- Add DONATE to the Start-tab command allowlist; it was blocked there
  with "No drawing open" while REPORT/CHANGELOG/ABOUT were allowed, so
  the Donate button did nothing on the welcome screen.
- Implement sys::open_url on wasm via web_sys window.open(_blank) — it
  was a no-op stub, so Donate, Release Notes and Send Feedback opened
  nothing on the web. The click is a user gesture, so the pop-up blocker
  allows it.

Native + web build.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/commands.rs

@@ -36,7 +36,7 @@ impl OpenCADStudio {
             && !matches!(
                 cmd,
                 "NEW" | "OPEN" | "EXIT" | "QUIT" | "REPORT" | "CHANGELOG" | "ABOUT"
-                    | "PLUGINS" | "PLUGINMANAGER"
+                    | "PLUGINS" | "PLUGINMANAGER" | "DONATE"
             )
         {
             self.command_line