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]>

Cargo.lock

@@ -39,6 +39,7 @@ dependencies = [
  "truck-shapeops",
  "ttf-parser",
  "ureq",
+ "web-sys",
  "windows-sys 0.61.2",
  "winresource",
 ]