Hide the native title bar and let the TopBar own the window
chrome — the Electron `titleBarStyle: 'hidden'` recipe:
- macOS keeps a real `NSWindow` (rounded corners, shadow,
edge-resize, key-window responsiveness) with the title bar made
transparent + emptied; the native traffic-light buttons stay,
pushed down via casement's `with_traffic_light_inset` to centre
in the 40 px TopBar. Windows / Linux drop decorations and the
TopBar paints its own close / minimise / maximise dots.
- The TopBar reserves a left inset for the controls; it collapses
in macOS fullscreen (native lights hide), tracked by a
per-frame `window.fullscreen()` poll. `window_control_at`
returns `None` on macOS so a fullscreen click on a left-edge
app icon can't trigger a window control.
- A press on the TopBar's blank area drags the window.
TopBar chip also gains one brand icon per connected agent + an
`N agent[s] · M MCP` status (new `topbar.agentPlural` across all
15 locales).
Bumps the vendor/casement submodule to 5ad98f1c.
bb4a989709