refactor(windows): evaluate hoisting ghostty_app_t ownership

[deblasis]

ghostty_app_t is currently owned per-window. MainWindow constructs a
GhosttyHost that calls ghostty_app_new, so each window runs its own
libghostty app.

The macOS apprt has the inverse topology: one Ghostty app handle at the
app level, per-window surfaces registered against it. libghostty's C API
is designed for one app per process.

Two ways to align:

a) Full refactor. Move ghostty_app_t ownership out of GhosttyHost into
a dedicated app-level service. Per-window hosts hold only the surface
dictionary and callback delegates.

b) Minimal hoist. App.xaml.cs owns the single ghostty_app_t. Each
MainWindow constructs a GhosttyHost that wraps the shared handle plus
a per-window surface dictionary.

Which shape is more appropriate as app-level coordination surfaces appear
(command palette, global config reload, inspector, activation)?