feat: make ECR login TUI-first and document the TUI-first product rule

[youngjinjung-linq]

Summary

Implements #178: unic's product posture is TUI-first, and ECR login was the one service capability still framed as CLI-first.

• New ECR Login Helper feature in the TUI under the ECR service: resolves the private registry URI for the active context and shows copyable Docker and Podman login commands. c copies the Docker command, p the Podman command, r re-resolves.
• unic ecr login stays unchanged as a secondary scripting/automation helper, and README now frames it that way.
• README gains a Product Principles section stating the TUI-first rule.
• CONTRIBUTING's PR checklist gains a TUI-first review item so future service features default to a TUI entry point.

Testing

• go test ./... passes, including new tests for the login-resolved message/screen transition, view content, esc navigation, and the catalog entry.
• make build passes.

Closes #178

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📥 Commits

Reviewing files that changed from the base of the PR and between 744b139 and 1b8e58d.

📒 Files selected for processing (10)

• CONTRIBUTING.md
• README.md
• internal/app/app.go
• internal/app/help.go
• internal/app/messages.go
• internal/app/screen_ecr.go
• internal/app/screen_ecr_login_test.go
• internal/domain/catalog.go
• internal/domain/catalog_test.go
• internal/domain/model.go

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[YoungJinJung]

Recreated as #256 from an in-repo branch under the maintainer account; this PR is superseded.