feat: Add configurable blank line detection and content splitting control

[uho-33]

Summary

This PR adds two new features to improve page break control:

• --blank-ratio: Makes blank line detection configurable, allowing for noisy images
• --no-split-content: Prevents content blocks from being split across pages

Details

Blank Line Detection (--blank-ratio)

• Default (0.0): Strict behavior - all pixels must be > 250 brightness
• With value > 0: Allow specified ratio of non-blank pixels (e.g., 0.1 = 10% tolerance)
• Particularly useful for images with compression artifacts or subtle backgrounds

Content Splitting Control (--no-split-content)

• Default: Content may be split at page boundaries if no gap is found
• With flag: Forces splits to occur only at content gaps, never cutting through content
• Includes helpful error handling when content blocks are too tall for selected page size

Usage Examples

# Allow 15% non-blank pixels in "blank" lines
scrollshot2pdf image.png --blank-ratio 0.15

# Prevent content from being split across pages
scrollshot2pdf document.png --no-split-content --page-size legal

[Copilot]

Pull Request Overview

This PR introduces two key features to enhance page break control in scrollshot2pdf: configurable blank line detection and content splitting prevention.

• Adds --blank-ratio parameter to allow tolerance for noisy pixels in blank line detection
• Adds --no-split-content flag to prevent splitting content blocks across pages
• Includes comprehensive error handling and documentation for the new features

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
scrollshot2pdf.py	Implements the core functionality for both new features with parameter validation and error handling
README.md	Documents the new options with usage examples and detailed explanations
CHANGELOG.md	Records the feature additions for version 0.1.2

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