feat(ui): OCS Web button, prefilled feedback, drop Start About button

- Add an "OCS Web" button (bright ribbon-blue, desktop only) to the Start
  page that opens the web build at hakanseven12.github.io/OpenCADStudio.
- Send Feedback now opens the GitHub issue with the body pre-filled:
  app version + platform (OS/arch on desktop, browser user-agent on web),
  via sys::platform_info + sys::percent_encode.
- Remove the redundant About button from the Start page (ABOUT command /
  menu still opens it).

Native + web build; 75 tests pass.

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

Cargo.toml

@@ -87,6 +87,7 @@ js-sys = "0.3"
 # window.open (external URLs) + Blob/anchor file downloads (Save).
 web-sys = { version = "0.3", features = [
     "Window",
+    "Navigator",
     "Document",
     "Element",
     "HtmlAnchorElement",