feat: add filter to context switcher

[YoungJinJung]

Summary

When there are many contexts in the context switcher (C key), it becomes difficult to find the desired one. Add / key filter support using the same keyword matching pattern as EC2/RDS/Route53 list screens.

Scope

Filter Logic

• Add ctxFilterInput, ctxFilterActive, filteredCtxList state fields
• Add FilterText() method to ContextInfo (match against name, profile, region)
• Add applyCtxFilter() function following existing filter pattern
• Toggle filter mode with / key in context picker

UI

• Show filter bar when active (same style as EC2 instance filter)
• Render filtered list instead of full list
• Show / : filter in bottom key binding hint

Docs

• Update README key bindings table

Reference

• Existing pattern: EC2 updateInstanceList() filter in screen_ec2.go
• Files: internal/app/screen_context.go, internal/app/app.go, internal/config/config.go