[feature]: Render Mermaid diagrams in the rich text editor (code block live preview)

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Summary

Render Mermaid diagrams in the rich text editor (work item descriptions, comments, pages): a code block with language mermaid shows a live-rendered diagram preview alongside the still-editable code — the same UX as GitHub/Notion.

Why should this be worked on?

Engineering teams keep architecture diagrams, flows and sequence charts next to the work item they describe. Today a ```mermaid block in Plane renders as plain code, so teams paste screenshots instead — un-editable, un-versioned, and quickly stale. Native rendering keeps diagrams as text: diffable, copy-pasteable, and editable in place.

Implementation notes (working PR attached):

• Zero schema change: builds on the existing code-block node (language-mermaid class), so Markdown round-trip, HTML export and yjs collaborative editing all keep working untouched; server-side HTML sanitization already allows language-* classes.
• mermaid (~350KB gzip) is loaded via dynamic import() only when a mermaid block is on screen — zero cost otherwise; SSR-safe.
• Diagram follows the editor's light/dark theme; render errors show the source plus the parser message instead of breaking the editor; rapid typing is debounced.
• Read-only surfaces (peek overview, space public pages) render the diagram automatically via the same node view.

Start by locating the existing code-block node and its language-mermaid handling in the rich text editor, then trace the node view used by read-only surfaces such as peek overview and public pages. Done means Mermaid blocks show editable code beside a themed live preview, render errors preserve the source and parser message, and rapid typing does not break the editor.