Add a graceful Shutdown action alongside Proxmox's Start/Restart/Stop

Stop maps to Proxmox's hard power-off (/status/stop) — fine for a
crashed guest, but risky for anything with a filesystem that'd rather
flush cleanly first. Proxmox exposes a separate /status/shutdown
endpoint that asks the guest to power itself down (ACPI event for a
VM, SIGTERM-then-wait for a container), so add it as its own action
rather than overloading Stop.

- New shutdownGuest() on the Proxmox adapter, calling /status/shutdown.
- The existing generic action route/loop already dispatches by
  ${action}Guest, so adding "shutdown" to that list was enough on the
  server side — no new route needed.
- New Shutdown button next to Restart/Stop on both the Integrations
  page's guest table and a Proxmox-linked server's detail page.
  Stop's confirm prompt now explicitly points at Shutdown as the
  gentler alternative.

Verified end-to-end against a mock Proxmox server: the shutdown call
hits /status/shutdown (never /status/stop) and is audit-logged as
shutdown_guest.

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

web/src/pages/Integrations.tsx

@@ -361,9 +361,10 @@ export default function Integrations({ user }: { user: CurrentUser }) {
     }
   }
 
-  async function guestAction(g: ProxmoxGuest, action: "start" | "stop" | "restart") {
+  async function guestAction(g: ProxmoxGuest, action: "start" | "stop" | "restart" | "shutdown") {
     if (!selectedId) return;
-    if (action === "stop" && !confirm(`Stop ${g.type === "qemu" ? "VM" : "container"} "${g.name}"?`)) return;
+    if (action === "stop" && !confirm(`Stop ${g.type === "qemu" ? "VM" : "container"} "${g.name}" immediately? Use Shutdown instead for a graceful power-off.`))
+      return;
     setActingOnGuest(g.vmid);
     setProxmoxError(null);
     try {
@@ -845,6 +846,13 @@ export default function Integrations({ user }: { user: CurrentUser }) {
                                       >
                                         Restart
                                       </button>
+                                      <button
+                                        className="btn btn-sm"
+                                        onClick={() => guestAction(g, "shutdown")}
+                                        disabled={actingOnGuest === g.vmid}
+                                      >
+                                        Shutdown
+                                      </button>
                                       <button
                                         className="btn btn-sm btn-outline-danger"
                                         onClick={() => guestAction(g, "stop")}