* fix(markdown): render default markdown with marked
- switch default frontend markdown rendering from react-markdown/remark-gfm to marked to avoid old WebKit parse failures from lookbehind regex literals
- sanitize marked HTML output with DOMPurify and preserve external link target and rel behavior
- remove default direct dependencies on react-markdown, remark-gfm, and rehype-raw while leaving classic unchanged
* fix(markdown): expand default markdown rendering support
- render default markdown with marked extensions for KaTeX formulas, page breaks, and common emoji shortcodes.
- sanitize KaTeX output with an explicit DOMPurify allowlist while preserving external link behavior.
- avoid overriding marked text rendering so lists and inline parsing keep their internal parser context.
* fix(markdown): render diagram code blocks in default UI
- add sanitized SVG rendering for flow and sequence diagram code blocks.
- size flow nodes from their labels and route edges from node anchors to prevent clipping.
- style diagram nodes, arrows, labels, and notes with theme-aware classes.
