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 <noreply@anthropic.com>

Cargo.toml

@@ -82,6 +82,8 @@ ureq = { version = "3", default-features = false, features = ["rustls"] }
 
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 console_error_panic_hook = "0.1"
+# Opening external URLs (Donate / feedback / release notes) via window.open.
+web-sys = { version = "0.3", features = ["Window"] }
 # Browsers without WebGPU (e.g. Firefox by default) need wgpu's WebGL2 backend;
 # `webgl` enables it. `fira-sans` embeds the default UI font — the web has no
 # system fonts, so without it all iced text (ribbon labels, command line) is