fix: replace underline with highlight color for TUI headings

[bug-ops]

Summary

• Terminal UNDERLINED modifier bleeds into empty cells beyond text content, causing visual artifacts on heading lines
• Replace bold+underline with bold+highlight color (orange) for markdown headings in TUI chat widget

Test plan

• cargo clippy -p zeph-tui -- -D warnings passes
• cargo nextest run -p zeph-tui --lib — 82 tests pass
• Visual verification in TUI with markdown headings

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             main     #342      +/-   ##
==========================================
- Coverage   79.23%   79.23%   -0.01%     
==========================================
  Files          99       99              
  Lines       25406    25403       -3     
==========================================
- Hits        20131    20128       -3     
  Misses       5275     5275              

Files with missing lines	Coverage Δ
crates/zeph-tui/src/widgets/chat.rs	49.58% <100.00%> (-0.32%)	⬇️

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