fix(ui): improve Markdown table fit and heading hierarchy #739

Summary

Improve the readability of the high-frequency Markdown conversation surface.

Problem

In the live Electron app on current main:

• the Markdown content column is 680px wide
• an ordinary three-column table resolves to about 730px
• table cells use 16px horizontal padding on each side
• body text is 13px/19.5px while H2 is about 16px/20px

As a result, common summary tables require horizontal scrolling even when their content is not intrinsically wide. Heading hierarchy in long answers also relies more on weight than on size and spacing.

Refs #546

Scope

• reduce unnecessary table-cell width
• improve wrapping for prose while preserving paths and code identifiers
• strengthen H2/H3 hierarchy without globally inflating application UI density
• preserve semantic table markup and keyboard scrolling

Acceptance criteria

• A representative three-column summary table fits the 680px conversation column without horizontal scrolling.
• Intrinsically wide tables remain horizontally scrollable.
• Long paths, code identifiers, Chinese copy, and mixed Chinese/English content remain readable.
• H2/H3 sections are clearly distinguishable from body copy in long answers.
• Table headers, cells, inline code, code blocks, links, and lists remain visually coherent.
• Light/dark themes and 1280px/990px viewports are checked.
• The invariant is covered by computed-style or text-contract tests, with screenshot coverage for the exposing viewport.

Out of scope

• changing the global application font scale
• redesigning the Markdown renderer or syntax support