refactor: decompose screen_reachability.go updateConfig() nested switch

[nathanhuh]

Summary

`updateConfig()` in `internal/app/screen_reachability.go` is a 68-line function with 3-level nesting handling 13 key cases that manage protocol selection, port input, IP input, and form navigation simultaneously. Tracing a single key event requires reading the entire function.

Details

• Switch-within-switch-within-switch structure makes control flow hard to follow
• All 13 input cases live in one function, mixing orthogonal concerns
• Adding a new form field requires understanding all existing cases

Checklist

• Extract `handleProtocolInput(msg tea.KeyMsg) (reachabilityModel, tea.Cmd)`
• Extract `handlePortInput(msg tea.KeyMsg) (reachabilityModel, tea.Cmd)`
• Extract `handleIPInput(msg tea.KeyMsg) (reachabilityModel, tea.Cmd)`
• Reduce outer `updateConfig()` to a dispatcher that routes to the appropriate handler based on current form focus state
• Verify existing reachability tests still pass

References

• `internal/app/screen_reachability.go:415-483`

Raised from senior code review (2026-05-12).