Consolidate SetQA AI Q&A renderer onto shared MarkdownView pipeline (DRY + mermaid parity)

[cgbarlow]

Background

SetQA.svelte maintains its own divergent local renderMarkdown (lines 618-624, used at lines 771 and 847 for completed and streaming AI answers). This duplicates the canonical pipeline in frontend/src/lib/components/markdownHelpers.ts and violates protocol #13 (DRY) in docs/protocols.md.

After v5.7.0 (ADR-149) the canonical MarkdownView pipeline gains Mermaid diagram rendering. The AI Q&A panel does not inherit that automatically because of the duplication.

Behavioural differences today

Capability	Shared MarkdownView	SetQA local
iris:// link click-through	Yes	No
Image rendering with src allowlist	Yes	No
URL-scheme allowlist	Default + iris:	Whitelist-based tag allowlist
Mermaid diagrams (post-v5.7.0)	Yes	No

Proposed change

Replace SetQA's local renderMarkdown with an import from markdownHelpers.ts and wire runMermaidIn(answerEl, currentTheme) after each {@html} injection (both completed and streaming answer containers). Verify streaming-partial mermaid degrades gracefully (incomplete fence renders as the small error placeholder until the closing fence arrives).

Acceptance criteria

• AI Q&A answers render mermaid diagrams (parity with /views/[id] Text views).
• AI Q&A answers honour iris:// link clicks (jump to diagram/element).
• AI Q&A answers respect the canonical image-src allowlist.
• No regression in existing AI Q&A rendering tests; new test asserts identical HTML between SetQA answer and MarkdownView for the same source.
• Streaming answers handle in-progress mermaid fences without crashing the panel.

References

• ADR-137 (text diagram + shared markdown renderer)
• ADR-149 (mermaid in markdown — v5.7.0)
• Protocol feat: Collections, multi-set AI context (v2.7.4) #13 (DRY) in docs/protocols.md
• frontend/src/lib/components/SetQA.svelte lines 618-624, 771, 847
• frontend/src/lib/components/markdownHelpers.ts

Out of scope for this issue

This issue covers the renderer fold only. Further AI-answer-specific UX (e.g. mermaid-block copy-to-clipboard, diagram fullscreen) belongs in its own follow-up.