Change modify/remove logic to auto-select single transfer with pair_id=0

When --pair-id is not specified:
- If there's exactly one transfer to the recipient with pair_id=0, auto-select it
- If there's one transfer with pair_id!=0, error with existing pair_id shown
- If there are multiple transfers, error asking to specify --pair-id

Also fix validation order: validate_pair_id_should_be_given() must be called
before validate_existence() to show correct error messages.

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>