feat(renderer): Mermaid diagram rendering with dark theme (M10.7)

- Lazy-loads Mermaid.js on first ```mermaid block detected
- Dark theme with Bitcoin orange accent colors
- Cached renders to avoid re-rendering on scroll
- Error display inline without crashing app

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/app/package.json

@@ -24,6 +24,7 @@
     "katex": "^0.16.33",
     "leaflet": "^1.9.4",
     "markdown-it": "^14.1.1",
+    "mermaid": "^11.12.3",
     "pdfjs-dist": "^5.5.207",
     "pinia": "^3.0.4",
     "plyr": "^3.8.4",