feat(start): replace info cards with a clickable intro video

Swap the two "Free and open source" / "Community driven" text cards on the
start page for the intro-video thumbnail with a play badge; clicking it opens
the video in the system browser (new Message::OpenUrl → sys::open_url). The
bundled thumbnail Handle is built once via OnceLock — Handle::from_bytes mints a
fresh unique id per call, so rebuilding it per view re-decoded and re-uploaded
the JPEG every frame and the image only appeared after a long delay.

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

src/app/update/mod.rs

@@ -208,6 +208,11 @@ impl OpenCADStudio {
 
             Message::OpenPathPicked(None) => Task::none(),
 
+            Message::OpenUrl(url) => {
+                crate::sys::open_url(&url);
+                Task::none()
+            }
+
             Message::OpenRecent(path) => {
                 // Recents are read from disk every save → the path may be
                 // stale. Skip silently if the file no longer exists; the