Prototype Mermaid embeds in markdown previews

[dankarization]

What

• detect fenced code blocks with language mermaid inside markdown file previews
• convert them into TL_pageBlockEmbed blocks
• render them inline via the existing ArticleViewer embed WebView path
• add a GitHub Actions workflow that builds a standalone debug APK for quick testing

Validation

• built and installed the Android test APK from this branch
• opened markdown documents with Mermaid blocks from Saved Messages
• verified inline preview inside the article sheet and full article viewer rendering on-device
• verified a longer markdown file with multiple Mermaid blocks and plain text preserved before, between, and after diagrams

Screenshots

Notes

• prototype only
• currently loads Mermaid from CDN inside the embed HTML
• height is reported back through TelegramWebviewProxy.postEvent("resize_frame", ...)
• intended to validate the UX path before discussing vendoring and offline policy