feat(web): render Mermaid diagrams in markdown

[jdalmeida]

What Changed

T3 Code now renders fenced Mermaid diagrams in chat messages and rendered Markdown file previews.

The diagram viewport supports pointer controls, keyboard controls, bounded pan, 50–300 percent zoom, and a reset action.

Why

Markdown diagrams appeared as source code. This made agent plans and architecture notes harder to read.

A stable Markdown component prevents diagram flicker when thread data changes. The viewport limits zoom and pan positions.

Fixes #4571

UI Changes

Before

After

Interaction

Watch the zoom, pan, and reset controls

Checklist

• This PR is focused on Mermaid rendering.
• I explained what changed and why.
• I included before and after images.
• I included a video for the zoom and pan controls.

About 790 changed lines come from the generated lockfile update for Mermaid.

Validation

• pnpm exec vp test run apps/web/src/components/MermaidDiagram.test.ts
• pnpm exec vp lint apps/web/src/components/MermaidDiagram.tsx apps/web/src/components/MermaidDiagram.test.ts apps/web/src/components/ChatMarkdown.tsx apps/web/src/components/FileMarkdownViewer.tsx
• pnpm --filter @t3tools/web typecheck
• pnpm --filter @t3tools/web build
• Local web test with an isolated T3 home and one new GPT-5.6-Sol thread.

Model: GPT-5.6 Codex

Harness: T3 Code Codex harness

Note

Render Mermaid diagrams in chat markdown with pan/zoom support

• Adds a MermaidDiagram component (MermaidDiagram.tsx) that renders mermaid code fences as interactive diagrams with toolbar controls for zoom in/out and reset.
• Diagrams render in strict security mode, respect the current light/dark theme, and serialize renders via a global queue to avoid concurrent conflicts.
• Introduces MarkdownRenderContext in ChatMarkdown.tsx to pass diffThemeName, isStreaming, and resolvedTheme to child renderers without prop drilling.
• Mermaid code fences are only rendered as diagrams when not streaming; during streaming they fall back to standard Shiki-highlighted code blocks.
• Risk: adds the mermaid package (~11.x) as a dependency, which is dynamically imported at render time.

^{Macroscope summarized a245ae8.}

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[macroscopeapp]

Approvability

Verdict: Needs human review

This PR introduces a new feature with ~400 lines of new component code for rendering Mermaid diagrams with interactive pan/zoom controls. New user-facing capabilities with substantial new logic warrant human review.

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