fix(main): replace recursion with return, add input validation

[tatenda-source]

Summary

• Replace infinite main() recursion with return on invalid input
• Wrap int(question(...)) calls in try/except ValueError
• Use sys.executable instead of hardcoded "python" for cron subprocess
• Validate account exists before use in affiliate marketing flow
• Fix incorrect error() calls passing "red" as second arg

Closes #223

Test plan

• Run python src/main.py and enter non-numeric input at menus
• Select invalid account numbers and verify graceful handling
• Test affiliate marketing flow with non-existent Twitter UUID

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