feat: Remove starting_idx parameter (closes #45)

[EmersonFras]

Summary

This PR removes the starting_idx parameter (-x) from the command line arguments and internal logic.

Previously, starting_idx was used to manually resume interrupted downloads. With the recent addition of check_existing_images (introduced in PR #43), the tool now automatically detects existing files in the output directory and filters them out of the download queue. Consequently, the manual index parameter is redundant and complicates the validation of expected image counts.

Changes Made

• Removed Argument: Deleted the -x / --starting-idx argument definition from parse_argsin __main__.py.
• Updated Main Logic: Removed the starting_idx argument from the check_existing_images function call.
• Updated Download Calls: Removed the starting_index parameter from both download_images calls (standard and downsampled paths).

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Closes #45

[egrace479]

nicely done!