Issue for this PR

Closes #

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds markdown preview mode with Code/Preview toggle tabs and Mermaid diagram rendering to the file viewer.

File viewer changes (packages/app/src/pages/session/file-tabs.tsx):

• Detects markdown files by extension (.md, .markdown, .mdx)
• Shows Code/Preview segmented toggle tabs (absolute positioned top-right) for markdown files only
• Preview mode renders markdown using the existing <Markdown> component
• Source mode preserves syntax highlighting, line selection, comments, and search
• Scroll position is preserved when switching between Code and Preview

Markdown rendering changes (packages/ui/src/components/markdown.tsx):

• Extracts ```mermaid code blocks from source before marked.parse() to prevent shiki from intercepting them
• After morphdom patches the live DOM, dynamically imports mermaid and renders SVG diagrams directly into the container
• Mermaid blocks are rendered with securityLevel: "strict" and dark theme
• Added SVG-related tags and attributes to DOMPurify allowlist

Marked config (packages/ui/src/context/marked.tsx):

• Removed unused Token import

How did you verify your code works?

• Typecheck passes for both packages/ui and packages/app
• Vite build succeeds (mermaid is code-split into its own chunk)
• Opened a .md file — Code/Preview tabs appear
• Clicked Preview — markdown renders with headings, code blocks, links, LaTeX
• Mermaid diagrams (flowchart, sequence, gantt) render as SVG
• Clicked Code — returns to raw source view, scroll position preserved
• Opened a non-markdown file — no tabs shown, behavior unchanged

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR