ui+server: schedule next-run / last-run on dashboard + schedules tab

P2R-14. New store.LatestJobBySchedule query (per-schedule fired job).
Schedules-tab handler computes next-fire from cron + last-fire from
the jobs table per row. Schedules table grows two columns; dashboard
host row prepends 'next 12h ago/from now' to the existing last-backup
line when a single covering schedule is the run-now candidate.

Embeds store.Schedule into scheduleRow so existing template field
references keep working without bulk renames.

internal/server/http/ui_handlers.go

@@ -124,6 +124,9 @@ type dashboardHostRow struct {
 	// match — in that case the row shows "Open →" instead of a Run-now
 	// button (the operator picks per-group from the host detail).
 	RunAllScheduleID string
+	// NextRun is the next-fire time of RunAllScheduleID (when set),
+	// computed server-side from its cron. nil otherwise.
+	NextRun *time.Time
 }
 
 // pickRunAllSchedule returns the ID of the single schedule whose
@@ -203,6 +206,17 @@ func (s *Server) handleUIDashboard(w stdhttp.ResponseWriter, r *stdhttp.Request)
 			slog.Warn("ui dashboard: list schedules", "host_id", h.ID, "err", serr)
 		}
 		row.RunAllScheduleID = pickRunAllSchedule(scheds, groups)
+		if row.RunAllScheduleID != "" {
+			for _, sc := range scheds {
+				if sc.ID == row.RunAllScheduleID {
+					if parsed, perr := cronParser.Parse(sc.CronExpr); perr == nil {
+						n := parsed.Next(time.Now().UTC()).UTC()
+						row.NextRun = &n
+					}
+					break
+				}
+			}
+		}
 		rows = append(rows, row)
 	}