refactor: extract shared LaTeX utility functions into a common module

Summary

convert_markdown_formatting_to_latex and _escape_remaining_latex_chars are duplicated in two files:

• app.py (lines 591, 640)
• resume_generator_latex.py (lines 39, 86)

These should be extracted into a shared utility module to prevent them from drifting out of sync.

What to change

1. Create a new file latex_utils.py in the project root.
2. Move convert_markdown_formatting_to_latex and _escape_remaining_latex_chars into it.
3. Update app.py to import from latex_utils.py instead of defining them inline.
4. Update resume_generator_latex.py to import from latex_utils.py instead of defining them inline.
5. Verify no other files duplicate these functions.

Acceptance criteria

• Both functions live in a single latex_utils.py module
• app.py and resume_generator_latex.py import from latex_utils.py
• No duplicate definitions remain
• Existing tests pass (cd resume-builder-ui && npm test for frontend; backend tests if applicable)
• PDF generation still works correctly via both entry points (app.py and resume_generator_latex.py)

Context

Flagged in PR #244 review. The functions were added to resume_generator_latex.py as part of the LaTeX escaping fix, mirroring the existing implementation in app.py.