feat: Add EC2 Security Group browser (M3.5)

[nathanhuh]

Summary

Add support for browsing and managing EC2 Security Groups in the TUI.

Details

• List/filter security groups by VPC, name, or ID
• View inbound and outbound rules with protocol, port range, source/dest CIDR or SG reference
• Add / Delete / Modify rules with confirmation before apply
• Drill-down from VPC view to associated security groups

Checklist

• Add SecurityGroup service/feature constants in internal/domain/model.go
• Register in catalog via internal/domain/catalog.go
• Implement ListSecurityGroups() in internal/services/aws/sg.go
• Implement sg_model.go with SecurityGroup, InboundRule, OutboundRule models
• Implement rule add/delete/modify operations with AuthorizeSecurityGroupIngress/Egress and RevokeSecurityGroupIngress/Egress
• Add SecurityGroupList, SecurityGroupDetail screens in internal/app/
• Add confirmation screen before rule modifications
• Write tests