feat(markdown): Render Mermaid diagrams in the preview (#82)

Fenced ```mermaid blocks in the Markdown preview render as diagrams. mermaid (12.1.0) is loaded lazily, only when a preview contains a diagram, with securityLevel "strict"; colours come from the theme tokens and diagrams re-render on theme changes. Syntax errors show inline under the source. Rendered diagrams are cached by theme + source, so editing elsewhere in the file doesn't redraw them.

Fixes #79

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

package.json

@@ -52,6 +52,7 @@
     "emmet": "2.4.11",
     "immer": "^11.1.21",
     "marked": "^18.1.1",
+    "mermaid": "12.1.0",
     "react": "^19.1.0",
     "react-dom": "^19.1.0",
     "react-resizable-panels": "^4.14.6",