add node_exporter host vitals + quickshell HUD

Prometheus node_exporter enabled on every host, plus a quickshell widget
(SUPER+CTRL+V on terra) to view live CPU/mem/disk/net/uptime without a
separate dashboard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FHr5ug9pu8q4XPrRkFnzJ

services/monitoring/node-exporter.nix

@@ -0,0 +1,9 @@
+{ ... }:
+
+# Prometheus node_exporter — host vitals (CPU/mem/disk/net/uptime) for the
+# homelab dashboard. No openFirewall needed: tailscale.nix already trusts
+# tailscale0, so :9100 is reachable over the tailnet and blocked on every
+# other interface (LAN, public) without any extra rule here.
+{
+  services.prometheus.exporters.node.enable = true;
+}