api.host.features.sidePanel now says whether this plugin may offer the
app-side-panel (ui:side-panel granted), not only that the webmail has
it. A plugin enabled before an update added the permission can then say
so instead of offering a switch that does nothing. The host computes the
assistant's flags per plugin for a frame's init and for each push.
plugin-host-features.test.ts: the host bridge assertions take the
assistant's flags as the test plugin sees them (it lacks ui:side-panel,
so sidePanel is false there).