style(tui): distinguish markdown h1 headings

[kitlangton]

Summary

• Add italic and underline styling to Markdown H1 headings in the TUI syntax theme.
• Keep generic Markdown headings and H2-H6 styling unchanged so table headers do not inherit H1 styling.

Verification

• bunx prettier --write src/cli/cmd/tui/context/theme.tsx
• bun typecheck from packages/opencode
• bunx oxlint packages/opencode/src/cli/cmd/tui/context/theme.tsx (existing warnings, 0 errors)

Notes

• Root pre-push typecheck currently fails in packages/core with unrelated Response/Headers type errors, so this branch was pushed with the pre-push hook skipped after confirmation.