Let admins hide the Proxmox-link card per server

Not every registered server is a Proxmox VM/LXC -- bare-metal boxes
and other hosts had no reason to show a "link to Proxmox" option, but
it appeared unconditionally on every server's detail page.

New hideProxmoxLink column on servers (default false, so existing
behavior is unchanged until someone opts in). A "Not a VM? Hide this"
link in the card's header sets it; once hidden, a small "+ Show
Proxmox link options" link takes its place so it's still reachable,
not buried in a settings form. The card always shows regardless of
this flag once a server IS actually linked, so unlinking never becomes
unreachable by hiding the card out from under an active link.

Verified against a temp SQLite DB with real migrations: a new server
defaults to false, and toggling true/false both persist correctly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -54,7 +54,10 @@ All modules from the original plan are built:
   detail page. The detail page also has an **Admin Links** section
   (operator/admin to add/edit/remove) for bookmarking that server's own
   admin UIs — Dockge, Webmin, Cockpit, Portainer, or anything else reachable
-  by URL.
+  by URL. Since not every server is a Proxmox VM, an admin can hide the
+  "Proxmox link" card per server ("Not a VM? Hide this" / "+ Show Proxmox
+  link options") — it stays visible regardless once a server actually is
+  linked, so unlinking is always reachable.
 - **Tailscale**, **Proxmox**, **Synology**, **Semaphore**, **Gitea**,
   and **Docker** each get their own top-level page (backed by the
   matching integration) instead of living inside a shared Integrations