[Story]: Render artifact summaries as proper Markdown instead of plain text

[adoLime]

User Story

As a project member, I want AI-generated artifact summaries to render as properly styled Markdown (headings, bullets, code blocks, quotes), so that the structured sections "Key points / Decisions / What changed" are visually distinguishable instead of reading like a wall of plain text.

Context & Motivation

• Type: Functional (Knowledge Base UX)
• Parent epic: 04 EPIC - Knowledge Layer, RAG & Source Citation #53 (04 EPIC – Knowledge Layer, RAG & Source Citation)
• Teams: team:frontend (lead), team:ai (prompt sanity check)

Status quo: The AI service already prompts for Markdown with the sections ## Key points, ## Decisions, ## What changed, and the frontend renders the summary with ReactMarkdown. The Raw-View of an artifact uses markdown-body and looks correct.

Gap noticed: The Summary-View uses prose prose-sm dark:prose-invert classes but @tailwindcss/typography is not installed (Tailwind v4 setup, no plugin in package.json, no import in src/styles/index.css). So the prose classes have no effect, the Markdown is parsed but renders with default browser styling, looking like plain text.

Acceptance Criteria

• Summary-View in ArtifactViewerDrawer renders Markdown with visible styling for headings, bullets, code blocks, and quotes, matching the look of the Raw-View.
• Verify the local LLM actually returns the expected Markdown sections (## Key points etc.) on a real artifact; if the model
  frequently drops them, file a follow-up on the AI side.
• Light and dark themes both render correctly; state is conveyed by structure (headings, bullets), not color alone; keyboard focus stays visible; no regressions in streaming/loading/error states.

Technical Notes

• Quickest fix: replace prose prose-sm dark:prose-invert with markdown-body in ArtifactViewerDrawer.tsx:414 (reuses the already installed github-markdown-css).
• Alternative: install @tailwindcss/typography and activate it in src/styles/index.css for project-wide prose support.
• AI side: prompt at artifact_summary.py:138-148 already requests Markdown; only verify, do not change unless the model misbehaves.