refactor: decompose screen_reachability updateConfig nested switch

[youngjinjung-linq]

Summary

Implements #205: updateConfig in screen_reachability.go handled field navigation, protocol selection, character input validation, and submission in one nested switch.

• The key switch now dispatches to focused helpers: configMaxField() (single source of truth for the manual-IP extra field), adjustConfigProtocol(), deleteConfigChar(), appendConfigChar() (digit/IP charset validation unchanged), and submitConfig() (advance from the protocol field, run analysis from input fields).
• Behavior is unchanged; the duplicated maxField computation that appeared three times is gone.

Testing

• go test ./... passes; new unit tests cover max-field derivation, protocol clamping (including no-op off the protocol field), port/IP character validation with backspace, and enter-advance vs. analysis-start.
• make build passes.

Closes #205

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📥 Commits

Reviewing files that changed from the base of the PR and between 744b139 and dfeea19.

📒 Files selected for processing (2)

• internal/app/screen_reachability.go
• internal/app/screen_reachability_config_test.go

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[YoungJinJung]

Recreated as #263 from an in-repo branch under the maintainer account; this PR is superseded.