Draft: prototype Mermaid embeds for Markdown preview

[dankarization]

Summary

Prototype support for fenced mermaid blocks in Markdown preview.

This patch converts code fences with language mermaid into a clickable placeholder that opens an embedded preview overlay and attempts to render the diagram there.

Related issue: #30715

What this does

• detects fenced code blocks with info string mermaid
• prepares them as a markdown placeholder instead of a plain code block
• attaches an embedded HTML resource to the placeholder
• renders the diagram inside the existing markdown embed overlay
• reports preferred size back to the overlay so the preview can resize

Why draft

This is a prototype path, not a polished final implementation.

Known limitations:

• it loads Mermaid from an external CDN instead of a bundled local runtime
• it has not gone through the full Telegram Desktop build/test pipeline
• it does not yet include screenshot/golden tests for rendered output
• fallback behavior is intentionally minimal

Intent

The point of this PR is to show a concrete integration path using the existing markdown placeholder + embed overlay infrastructure, so the discussion can move from abstract feature request to a real patch shape.

If this direction makes sense, the next obvious steps would be:

• replace CDN loading with a vendored/runtime-approved Mermaid integration
• add proper rendering tests
• refine placeholder text and failure UX

[CLAassistant]

All committers have signed the CLA.