fix(monitor): write Proxmox's notification settings only while /etc/pve is mounted

- Nothing is written under /etc/pve unless it is the mounted cluster
  filesystem; a Monitor that starts while pve-cluster is being restarted
  waits for it and registers its webhook afterwards.
- The Monitor service is ordered after pve-cluster.service.
- The settings are written only when they have to change. The copies
  taken before a change are kept in
  /usr/local/share/proxmenux/backups/pve-notifications, and the ones left
  in /etc/pve by earlier versions are moved there.
- Uninstalling the Monitor removes its target and matcher from Proxmox.

Reported in #427.

web/messages/en/docs/settings/uninstall-proxmenux.json

@@ -19,7 +19,7 @@
     "items": [
       "<strong>Confirmation dialog.</strong> Yes/no — “Are you sure you want to uninstall ProxMenux?”",
       "<strong>Dependency removal checklist.</strong> Pre-selected list depends on install type (see table below). All boxes default to OFF — you opt in to removing each dependency.",
-      "<strong>Removal runs with a progress bar.</strong> Removes the venv (Translation install only), the <code>menu</code> launcher, the <code>/usr/local/share/proxmenux/</code> tree, the ProxMenux Monitor service if present, the dependencies you ticked, and finally restores the system file backups (see below).",
+      "<strong>Removal runs with a progress bar.</strong> Removes the venv (Translation install only), the <code>menu</code> launcher, the <code>/usr/local/share/proxmenux/</code> tree, the ProxMenux Monitor service if present together with the notification target it registered in Proxmox VE (<code>proxmenux-webhook</code> and its matcher), the dependencies you ticked, and finally restores the system file backups (see below).",
       "<strong>Completion dialog.</strong> Lists removed dependencies and exits."
     ]
   },