Refactor: Replace mutool with PDFBox for Poster PDF Splitting

Summary

Refactored the "Split for Poster Print" feature to use Apache PDFBox instead of mutool, eliminating the external binary dependency while maintaining full functionality and improving code maintainability.

Changes

Improved from PR 5682

Backend Refactoring

• Replaced mutool with PDFBox: Complete rewrite of PosterPdfController using pure Java/PDFBox implementation
• Removed external process execution: No more ProcessBuilder or shell command execution
• Vector graphics preserved: Content remains vector-based (no rasterization)
• Page normalization: Implemented CropBox-to-MediaBox normalization for consistent coordinate space handling
• Improved CropBox handling: Properly handles PDFs with non-zero CropBox origins and two-page spreads

Closes #5644

Checklist

General

• I have read the Contribution Guidelines
• I have read the Stirling-PDF Developer Guide (if applicable)
• I have read the How to add new languages to Stirling-PDF (if applicable)
• I have performed a self-review of my own code
• My changes generate no new warnings

Testing (if applicable)

• I have tested my changes locally. Refer to the Testing Guide for more details.

🌐 TOML Translation Verification Summary

🔄 Reference Branch: pr-branch

📃 File Check: en-GB/translation.toml

1. Test Status: ✅ Passed
2. Test Status: ✅ Passed
3. Test Status: ✅ Passed

✅ Overall Check Status: Success

Thanks @intelliking for your help in keeping the translations up to date.

Hi, @Frooodle Can you check my PR again? Thank you.

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This is a very similar tool to split-pdf-by-sections. Would it make sense to combine the tools in the frontend so that the user can select if they want to split the file into multiple files or keep it as a single file? Can the same endpoint be used for both tools?

@ConnorYoh Thanks for your review. As you mentioned these two tools are similar. The differences are multiple/single file option and unlike split-pdf-by-sections, new tool can split pages into regular paper size like A4 for print purpose. In my opinion, they shouldn't be combined. While they both divide pages, the user intent is different. In addition, the underlying PDF manipulation is different enough that combining them would require significant refactoring.

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