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/shares.go

@@ -39,6 +39,7 @@ type sharesVM struct {
 	NForm       nfsForm
 	ApplyErr    string // last Apply failure: config saved but not live
 	Squashes    []string
+	External    []shares.ExternalShare // shares of other services (drop-ins), read-only
 }
 
 func (s *Server) applyErr() string {
@@ -96,6 +97,7 @@ func (s *Server) sharesView(st sched.Status, form smbForm, nform nfsForm) shares
 			vm.NForm.Squash = "root"
 		}
 	}
+	vm.External = s.shm.External()
 	if u, err := s.shm.ListUsers(); err == nil {
 		vm.Users = u
 	} else {