feat: Add VPC Reachability Analysis (NetworkInsights)

[nathanhuh]

Summary

Add VPC Reachability Analysis support using AWS Network Insights. This was planned as part of M3.1 (VPC) but not yet implemented.

Details

• Create and run NetworkInsights path analysis between two resources
• Display reachability results (reachable/not reachable) with hop-by-hop path
• Show blocking components (security groups, NACLs, route tables) when not reachable
• Source/destination picker from existing EC2 instances, ENIs, or IP addresses

Checklist

• Implement CreateNetworkInsightsPath() and StartNetworkInsightsAnalysis() in internal/services/aws/vpc.go
• Implement GetNetworkInsightsAnalysisResults() with polling for completion
• Add ReachabilityAnalysis screen in internal/app/
• Add source/destination picker component
• Display hop-by-hop path visualization
• Write tests