Refactor: split app.go into per-screen files

[nathanhuh]

Problem

internal/app/app.go is 2,127 lines with 80+ fields on Model. Every screen's state, update logic, view rendering, and data loading lives in one file.

Plan

Split by screen/resource into separate files in internal/app/:

New file	Contents
screen_ec2.go	updateInstanceList, viewInstanceList, loadInstances, startSSMSession, applyFilter, EC2 msgs
screen_vpc.go	VPC + Subnet update/view/load methods, applyIPFilter
screen_rds.go	RDS update/view/load/poll methods, applyRDSFilter, confirm screen, RDS msgs
screen_route53.go	Route53 zone/record update/view/load/filter methods, Route53 msgs
screen_context.go	Context picker/add update/view/load/switch methods
messages.go	All *Msg structs
styles.go	Style vars + renderStatusBar, fitToHeight
app.go (remains)	Model struct, New, Init, top-level Update/View dispatch

Pure file split — no interface or logic changes.

[nathanhuh]

@Unic-bot: assign me

[github-actions]

@nathanhuh has been assigned to this issue.