Fix XLSX TTY check

[macrat]

Summary

• move isTerminal/isCygwinTerminal helpers above ConvCommand.Run
• adjust xlsx condition to use them
• consolidate TTY tests into conv_internal_test.go and cover all format/terminal combinations

Testing

• go test ./cmd/ayd -run TestConvCommand_TTYWarning -count=1
• go test ./cmd/ayd (fails: TestAydCommand_Run_permissionDenied)

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.89%. Comparing base (a64dc74) to head (aec323c).

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Additional details and impacted files

@@            Coverage Diff             @@
##            v0.17      #24      +/-   ##
==========================================
+ Coverage   87.80%   87.89%   +0.08%     
==========================================
  Files          72       74       +2     
  Lines        6429     6466      +37     
==========================================
+ Hits         5645     5683      +38     
+ Misses        645      644       -1     
  Partials      139      139              

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