fix(ui): restore markdown heading hierarchy in Desktop/web chat

[akinshaywai]

Issue for this PR

Closes#16046

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

In packages/ui/src/components/markdown.css, all heading levels (h1–h6) shared a single rule with the same font size (14px), same weight, and same colour. This meant headings were visually identical to each other and nearly indistinguishable from bold body text. Long structured responses — plans, summaries, step-by-step guides — were impossible to scan.

The fix splits the shared rule into per-level rules using design tokens that were already defined in the theme but unused:

• h1 → 20px (matches --font-size-x-large)
• h2 → 16px (matches --font-size-large)
• h3 → 14px, weight 700
• h4–h6 → 14px, weight 600

All levels now use var(--markdown-heading, var(--text-strong)) for colour. The --markdown-heading token (#9d7cd8) was already defined in the theme but never wired in. The fallback ensures custom themes without this token are unaffected.

Spacing is differentiated by level to keep the chat layout compact while making sections scannable.

How did you verify your code works?

Reviewed the existing token values in the theme to confirm font size and colour tokens map correctly. The change is CSS-only with no logic — the heading selectors are independent and do not affect any other component styles.

Screenshots / recordings

Before: all headings render at 14px, same weight as body text — no visual hierarchy.

After: h1 = 20px, h2 = 16px, h3 = 14px bold — clear hierarchy matching standard markdown conventions.

(Local screenshot pending — the change is a direct application of the existing --markdown-heading token and size tokens already in the design system.)

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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