Add an Overview page at / (top of the sidebar, filling the previously
blank root route) summarising the cluster: hosts connected, instances
running, users online, aggregate CPU/memory/disk usage, controller
uptime, a recent-errors panel (LogConsole), and a link to Grafana when
configured. Each card is gated by the relevant permission.
Adds a controller.grafana_url config field, read on the page via
ControllerConfigGetRequest, for the Grafana link.
Closes#928.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>