Adds PendingUpdatesIndicator to MainLayout's RailFooter, above .rail-user.
Self-gating on the Deployer/Administrator role claims, it polls the
process-wide single-flight IPendingUpdatesCache every 30s through PollGate
and renders four states: checking / up to date / pending (warning pill plus
an 'N instances · M sites' breakdown) / unavailable. Each half of the
breakdown links only for the role that can act on it (Topology => Deployer,
Sites => Administrator), and a site-scoped operator's totals are narrowed
through SiteScopeService so they never count a site they cannot see.
Claude-Session: https://claude.ai/code/session_01Y5ef34iUUMjmwnMpL9xgmB
Joseph Doherty