Shares of other services and RepoISO integration

- include SMB drop-ins from /etc/samba/smb.conf.d into the generated smb.conf
- keep smbd/nfs-server running while other services (/etc/exports.d, drop-ins) share,
  instead of disabling them when nasmon has no shares of its own
- list those shares read-only on /shares
- settings: RepoISO URL; poll its /api/integration/status, dashboard card, menu link,
  alerts for portal down, stale worker, failed jobs, unmounted publications

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/templates/layout.html

@@ -16,6 +16,7 @@
     <a href="/shares" {{if eq .Page "shares"}}class="on"{{end}}>Shares</a>
     <a href="/alerts" {{if eq .Page "alerts"}}class="on"{{end}}>Alerts{{if .Alerts}} <span class="badge crit">{{.Alerts}}</span>{{end}}</a>
     <a href="/settings" {{if eq .Page "settings"}}class="on"{{end}}>Settings</a>
+    {{if .RepoISO}}<a href="{{.RepoISO}}" target="_blank" rel="noopener" title="ISO library, repositories, Proxmox exports">RepoISO ↗</a>{{end}}
   </nav>
   <span class="refresh" id="refresh-state" title="auto-refresh"></span>
 </header>