Handle dynamic form submission and allow wkhtmltopdf path override

[mikerobinsonshow]

Summary

• send dynamic form data to the Flask backend via a new handleSubmit handler
• display PDF download link or server-side validation errors
• add placeholder in the template for result messages
• ensure server only validates submitted fields
• add unit tests for server-side validation
• gracefully handle missing wkhtmltopdf and return an error instead of crashing
• allow setting a manual wkhtmltopdf path via the WKHTMLTOPDF_CMD environment variable
• add unit test verifying that a custom wkhtmltopdf path is used

Testing

• python -m py_compile app.py
• python -m pytest

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https://chatgpt.com/codex/tasks/task_e_68b4a864d314832fabcb7d16b3e9205c