feat(pdf): improve PDF font subsetting process with subprocess and timeout handling

[awwaawwa]

This PR enhances the PDF creation process in BabelDOC with more robust font subsetting.

Changes:

• Add subprocess-based font subsetting with timeout protection to prevent hanging
• Create a dedicated subset_fonts_in_subprocess method that handles process management
• Implement proper error handling and logging for font subsetting failures
• Gracefully fall back to the original PDF when subsetting fails
• Add temporary file handling for input/output during the subsetting process
• Improve debug information writing with better error handling

Benefits:

• Prevents the application from hanging indefinitely during font subsetting
• Improves reliability by adding proper timeout handling (60 seconds)
• Maintains backward compatibility by falling back to original PDF when needed
• Enhances user experience by ensuring the PDF generation process completes

These changes make the PDF generation more robust, especially when dealing with complex fonts that might cause subsetting to hang or fail.

[awwaawwa]

MuPDF's subset_fonts will block all other threads under specific circumstances, so it is executed in a subprocess.