Viewer: style non-table markdown in exhibit bodies (headings, lists, blockquotes, code)

[EYH0602]

Background

Issue #32 shipped client-side markdown rendering for exhibit bodies. The
renderer (crates/ara-viewer/src/markdown.rs, render_exhibit_body) runs
pulldown-cmark, which parses all of CommonMark plus the tables and
strikethrough extensions. So an exhibit body can legitimately render headings
(# …), lists, blockquotes (> …), inline code, code blocks, <strong>,
<em>, horizontal rules, links, and images — not just tables.

Problem

Rendering ≠ styling. The CSS added in #32 (crates/ara-viewer/public/styles.css)
only styles two things inside .exhibit-body:

• .exhibit-body table / th / td — the data-table look (borders, padding,
  header shading).
• .exhibit-body { overflow-x: auto; max-width: 100% } — the scroll container.

Every other rendered element (h1–h6, blockquote, ul/ol/li, code,
pre, hr, strong/em, a) falls back to default/inherited browser styling.
Inline emphasis (bold/italic) and paragraphs look fine because they inherit the
body type, but headings, blockquotes, lists, and code blocks render unstyled
and won't match the viewer's design language.

Why it's currently low-impact

The corpus today is "captions + prose + tables" (per docs/hub-parity.md), so
exhibit bodies are mostly paragraphs, bold/italic, and tables — all already
covered. This only bites once exhibits start using headings, blockquotes, lists,
or fenced code.

Proposed fix

Add baseline .exhibit-body-scoped rules for the non-table elements, reusing the
existing design tokens (--ink, --muted, --line, --panel2, the mono font
already used by pre.diff / .recipe-body). Roughly:

• .exhibit-body h1..h6 — size/weight scale consistent with .detail-title.
• .exhibit-body blockquote — left border + muted text.
• .exhibit-body ul/ol — indentation, marker spacing.
• .exhibit-body code (inline) and .exhibit-body pre — mono font + --panel2
  background, mirroring pre.diff (styles.css:1179-1190).
• .exhibit-body a — link color + hover.
• .exhibit-body hr — --line rule.

Acceptance

• An exhibit body mixing a heading, a list, a blockquote, and a code block
  renders with styling consistent with the rest of the detail pane.
• Add/extend a web test (or a visual check) covering a non-table body.
• No change to the table styling or scroll behavior from RESULT: client-side markdown rendering of exhibit bodies (D4, hub-parity) #32.

Refs

• Renderer: crates/ara-viewer/src/markdown.rs
• Current exhibit CSS: crates/ara-viewer/public/styles.css (.exhibit-body*)
• Follow-up to RESULT: client-side markdown rendering of exhibit bodies (D4, hub-parity) #32 / PR fix(viewer): complete exhibit markdown rendering (styling, in-block scroll, link-scheme XSS guard) (#32) #45.