Render Mermaid diagrams in the pull request reader #1073

Qodo (and similar bots) put a ```mermaid fence in the PR overview Diagram section. The desktop pull request reader has no Mermaid renderer, so that fence shows as a plain code block.

Context

Follow-up from kurrent-io/kcap-cli#1029 / kurrent-io/kcap-cli#1073. GitHub-flavoured HTML and image loading land in that PR; Mermaid was out of scope.

Goal

Render mermaid fenced blocks in the GitHub flavour of MarkdownView as diagrams (or a deliberate fallback that is better than a raw fence), without changing the chat flavour.

Notes

• Needs a design choice: native renderer vs embedded engine, size limits, dark-theme styling, failure/fallback when the fence is invalid.