feat: add refresh key binding to EC2 SSM instance list

[YoungJinJung]

Summary

The EC2 SSM instance list does not reflect newly launched instances until the user navigates back and re-enters the screen. Add an in-place refresh capability.

Scope

Refresh Key Binding

• Add r key binding to instance list screen
• Reuse existing loadInstances() flow (Loading screen → refreshed list)
• Reset filter and selection index on refresh
• Ignore r during filter mode (treat as text input)

UI

• Show r: refresh in bottom key binding hint

Docs

• Update README key bindings table

Reference

• Existing pattern: RDS detail r key refresh / poll
• File: internal/app/screen_ec2.go