Description of Changes

Please provide a summary of the changes, including:

• What was changed
  Updated the values of split.desc.6, split.desc.7, and split.desc.8 in src/main/resources/messages_zh_CN.properties to correct the page numbers:

• Why the change was made
  The previous numbering was inconsistent and would have led to incorrect split outputs in the Chinese UI. This ensures that users splitting a document see the correct page ranges.

• Translation Method
  The correction of these translation strings was generated and verified using AI assistance.

Closes #3529

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

Documentation

• I have updated relevant docs on Stirling-PDF's doc repo (if functionality has heavily changed)
• I have read the section Add New Translation Tags (for new translation tags only)

UI Changes (if applicable)

• Screenshots or videos demonstrating the UI changes are attached (e.g., as comments or direct attachments in the PR)

Testing (if applicable)

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