feat: next run countdown in auto-cycle config

- auto_cycle.py: next_run_at now set to future timestamp (now + interval)
  instead of current time — was always showing wrong value
- Config.tsx: NextRunCountdown component shows live countdown (updates
  every second) + absolute local time, only visible when auto-cycle enabled

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/services/auto_cycle.py

@@ -744,9 +744,10 @@ def _scheduler_loop(stop_event: threading.Event):
             interval_hours = 3.0
 
         _current_status["interval_hours"] = interval_hours
-        next_run = datetime.utcnow().isoformat()
+        from datetime import timedelta
+        next_run = (datetime.utcnow() + timedelta(hours=interval_hours)).isoformat()
         _current_status["next_run_at"] = next_run
-        logger.info(f"[Scheduler] Next cycle in {interval_hours}h")
+        logger.info(f"[Scheduler] Next cycle in {interval_hours}h (at {next_run[:16]} UTC)")
 
         # Wait for the interval (or until stop is signalled)
         stop_event.wait(timeout=interval_hours * 3600)