Add start/restart/stop buttons to Proxmox-linked servers' detail page

The Proxmox start/stop/restart routes already existed (used by the
Integrations page's guest table) but weren't reachable from a server's
own detail page, even when that server was linked to a Proxmox guest.

Reuses the existing POST /api/integrations/:id/proxmox/nodes/:node/
:type/:vmid/{start,stop,restart} routes and the operator+ role gate
already enforced there — no server-side changes needed. Buttons only
render for hardware.source === "proxmox", mirroring the Integrations
page's running/stopped button-set logic, with the same confirm-before-
stop prompt for the non-reversible action.

Verified end-to-end against the real dev server with a mock Proxmox
HTTPS server: linked a server to a mock guest, confirmed the detail
endpoint's live status, then triggered restart/stop/start and
confirmed the mock actually received each action and every call was
audit-logged.

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

README.md

@@ -28,7 +28,8 @@ All modules from the original plan are built:
   overview opens a detail page with CPU/RAM/disk status, IP addresses, and
   matching DNS names (looked up from the DNS module's cache) — live from
   Proxmox for VM/LXC-backed servers, or from the agent's own hardware
-  report for everything else.
+  report for everything else. Proxmox-linked servers also get
+  start/stop/restart buttons right on the detail page.
 - **Integrations → Tailscale** — device list with online/authorized status,
   and authorize/deauthorize/remove actions; a live device-count widget.
 - **Integrations → Gitea** — repo list with each repo's last CI run status,