ui: Slice E — admin creds form + run-now buttons + repo health panel

- hostRepoPage gains AdminURL/AdminUsername/HasAdminPassword, Online,
  and StatsView (pre-dereferenced projection of host_repo_stats).
- loadHostRepoPage loads the admin slot (tolerating ErrNotFound),
  hub.Connected, and stats (tolerating ErrNotFound).
- renderRepoPage gains an adminErr parameter; all callers updated.
- handleUIAdminCredentialsSave / handleUIAdminCredentialsDelete added
  (form-POST handlers mirroring the repo-creds pattern, with audit).
- Routes /hosts/{id}/admin-credentials POST and /delete POST registered.
- Template: Admin credentials form after Connection, Run-now HTMX
  buttons after Maintenance, Repo health stats panel in right rail.
- Tests: 9 new tests covering rendering, disabled states, save/delete
  round-trips, audit rows, and idempotent delete.

internal/server/http/server.go

@@ -220,6 +220,9 @@ func (s *Server) routes(r chi.Router) {
 		r.Post("/hosts/{id}/repo/credentials", s.handleUIRepoCredentialsSave)
 		r.Post("/hosts/{id}/repo/bandwidth", s.handleUIRepoBandwidthSave)
 		r.Post("/hosts/{id}/repo/maintenance", s.handleUIRepoMaintenanceSave)
+		// Admin credentials form (separate slot for prune-capable user).
+		r.Post("/hosts/{id}/admin-credentials", s.handleUIAdminCredentialsSave)
+		r.Post("/hosts/{id}/admin-credentials/delete", s.handleUIAdminCredentialsDelete)
 		// Schedules tab + create/edit/delete forms.
 		r.Get("/hosts/{id}/schedules", s.handleUISchedulesList)
 		r.Get("/hosts/{id}/schedules/new", s.handleUIScheduleNewGet)