feat(manager): add Proxmox cluster link to Containers page

[horner]

Summary

Adds a Proxmox button to the Containers page header (/sites/:siteId/containers) that opens the Proxmox cluster web UI (e.g. https://os.mieweb.org:8006) in a new tab.

Details

• The button sits between Nodes and New container in the page header.
• The cluster URL is derived from the origin of a node's API URL in the container list, so no cluster host is hardcoded — it adapts to any deployment.
• The button only renders when a node API URL is available (new URL(apiUrl).origin), and silently hides if none can be parsed.
• Opens in a new tab with rel="noopener noreferrer" and an accessible aria-label ("Open Proxmox cluster web UI").

Testing

Verified locally via make dev: the button renders in the header and links to the cluster origin.

[runleveldev]

Clicking the node name already opens the node directly to the selected container. This feature is both redundant and less useful since it doesn't also navigate to the specific container.

[horner]

Where. No one knows where?

[horner]

There is no clickable way to get there from that screen.

[runleveldev]

In both views, you click the node name. I'm not against making this more prominent, but it is here.