Refactor CLI flow and support custom output directories

[fmolinagomez]

Summary

• refactor the CLI script into reusable helpers and add an --output-dir option while improving single card and page rendering flows
• update image processing utilities to honour custom output directories and expose a reusable slugify helper
• document the new option and add unit tests covering slug generation and image output directory handling

Testing

• pytest

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https://chatgpt.com/codex/tasks/task_e_68e3c63c2b3c832eafdf668c737737e1