The Mantine Indicator component (used by the Proxmox cluster health
widget for VM/LXC status dots) sits at z-index 200, which is higher
than the AppShellHeader's default 100. This caused the status dots
to bleed above the header when scrolling.
Set the header z-index to 201 so it stacks above Indicator content
without conflicting with modal/popover layers (300+).
Closes#6194