feat(fleet): add Pro fleet management features and container drill-down (#174)

Add fleet health summary cards, container-level drill-down, node sorting
and filtering, fleet-wide search, and critical node detection to the
Fleet View. Fix ProGate to wrap placeholder content and add error toasts.

docs/docs.json

@@ -35,6 +35,7 @@
           "features/global-observability",
           "features/host-console",
           "features/multi-node",
+          "features/fleet-view",
           "features/alerts-notifications"
         ]
       },