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

dotfiles/quickshell/shell.qml

@@ -7,6 +7,7 @@ import qs.widgets.notifications
 import qs.widgets.osd
 import qs.widgets.sidebar
 import qs.widgets.systray
+import qs.widgets.vitals
 
 Scope {
     // Left sidebar in the Slant (V6) style — toggle with SUPER CTRL S.
@@ -25,4 +26,7 @@ Scope {
 
     Notifications {}
     VolumeOsd {}
+
+    // Host vitals HUD — toggle with SUPER CTRL V.
+    Vitals {}
 }