Fix HTML diff line numbering when blank lines are stripped

[drogers0]

Summary

HTML diff line numbers become inaccurate when blank lines are removed before rendering.

Affected code

• src/clonehunter/reporting/html_reporter.py:180
• src/clonehunter/reporting/html_reporter.py:268

Problem

_strip_blank_lines() removes empty lines from compare text, but displayed line numbers still increment from original start_line.
This shifts line mapping and can mislead users when navigating source.

Expected

Preserve original line positions in rendered diff (or avoid stripping blanks before numbering).