feat: add ECR login helper

[YoungJinJung]

Summary

Add an ECR login helper so unic can generate and copy the correct container registry login command for the current AWS context.

Scope

• Derive the private ECR registry URI from the current context account and region
• Support Docker and Podman command variants
• Copy the generated login command to the clipboard
• Optionally show registry URI and follow-up push/pull snippets

Acceptance Criteria

• ECR exposes a dedicated login helper feature in the TUI or CLI
• The generated login command matches the current context account and region
• Docker and Podman variants are supported
• The command can be copied to the clipboard
• Tests cover command generation and registry URI resolution
• README is updated