feat: escape HTML

[rich-iannone]

This PR adds HTML-escaping of unformatted body cells whenever a table is rendered as HTML. This is whether the table is displayed in a notebook, shown via show(), or exported with as_raw_html(). Previously, only LaTeX output escaped unformatted cells but this change extends that to all output contexts.

Cells processed by any fmt_*() method are treated as trusted and pass through unchanged. The escaping also covers group labels (when derived from raw group_id rather than a formatted group_label) and summary stub labels.

A new fmt_passthrough() method gives users explicit control: escape=True (default) escapes special characters, escape=False passes raw HTML/LaTeX through, and pattern= decorates values without transforming them.

[codecov]

Codecov Report

❌ Patch coverage is 85.36585% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.71%. Comparing base (764038b) to head (ecd3fb0).

Files with missing lines	Patch %	Lines
great_tables/_formats.py	77.77%	4 Missing ⚠️
great_tables/_utils_render_latex.py	66.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #881      +/-   ##
==========================================
- Coverage   98.83%   98.71%   -0.12%     
==========================================
  Files          51       51              
  Lines        7696     7718      +22     
==========================================
+ Hits         7606     7619      +13     
- Misses         90       99       +9     

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