The traffic lights were pinned to the library strip's 40px band, so on
macOS the 65px map card could never share their line. The editor header
now tells the window its band height over a small method channel and
MainFlutterWindow re-centres the lights on it; they move back up when the
editor closes. With that, every platform centres the controls, the map
card, and the window buttons on the same line.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>