feat: Real-time cluster health monitoring and status checks

[rafaelrsantosti]

Description

Add continuous health monitoring for clusters instead of checking only when the page loads.

Current Behavior

• Gateway API availability is only checked when viewing the clusters list
• No periodic health checks for cluster connectivity
• Status may be outdated if cluster goes down after initial check

Expected Behavior

• Periodic health checks for all registered clusters
• Real-time status updates for:
  • Cluster connectivity (API reachable)
  • Gateway API availability
  • Node status (Ready/NotReady count)
• Visual indicator when cluster status changes
• Optional notifications when a cluster becomes unhealthy

Acceptance Criteria

• Cluster status reflects real-time health
• Gateway API availability is checked periodically
• Node status is monitored
• Users can see when a cluster goes offline
• Status updates without page refresh