ux(admin/system): give the host info strip a "Host" section head

The OS/Kernel/Uptime/CPU/Swap strip was the only section on the System
page rendered without a .sys-section-head, so it had no title and butted
directly against the charts above it (no top spacing). Add a "Host" head
— matching the Docker / Per-app pattern — which supplies both the label
and the section's 26px top margin. "Host" rather than "System" since the
page H1 is already "System".

Signed-off-by: librelad <librelad@digitalangels.vip>

containers/libreportal/frontend/js/components/admin/admin-system.js

@@ -259,6 +259,7 @@ class AdminSystem {
             </div>`;
 
         const infoStrip = `
+            <div class="sys-section-head"><h2>Host</h2></div>
             <div class="sys-strip">
                 ${this.stat('OS', info.os || '—')}
                 ${this.stat('Kernel', info.kernel || '—')}