feat(web): render Mermaid diagrams in chat messages

[luzhongqiu]

Summary

This PR adds Mermaid rendering support to Kimi Web chat messages.

Previously, Mermaid fenced code blocks in chat were displayed as plain code blocks, so flowcharts and other Mermaid diagrams could not be rendered inline in the web UI.

What Changed

• detect mermaid fenced code blocks in the existing chat markdown/code-block rendering path
• render Mermaid diagrams inline with a dedicated client-side Mermaid component
• keep existing code block affordances such as copy/download for Mermaid content
• add a safe fallback that shows the Mermaid source when rendering fails
• add a preview screenshot for the rendered result

User Impact

Users can now paste or receive Mermaid flowcharts directly in Kimi Web chats and see them rendered as diagrams instead of raw code.

Screenshot

Validation

• npm --prefix web run typecheck
• npm --prefix web run build
• manual verification in uv run kimi web

Notes

• npm --prefix web run lint still reports two pre-existing unrelated Biome issues in web/src/features/chat/components/approval-dialog.tsx and web/src/features/chat/components/session-files-panel.tsx.

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[chatgpt-codex-connector]

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

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