feat: Mermaid.js integration #20-29 — full diagram support

- CDN: mermaid@11 from jsdelivr
- mermaid-viewer.js: init, render, templates, zoom/pan, fullscreen, focus
- All diagram types: flowchart, sequence, class, state, ER, journey, gantt,
  pie, gitGraph, requirement, mindmap, timeline, quadrant, C4 (x4), beta types
- viewer.js: auto-render Mermaid blocks after markdown rendering
- utils.js: live preview panel in editor (debounced 500ms)
- app.js: initMermaid + setupFocusMode on startup
- CSS: container, controls, error, fullscreen overlay, focus panel, live preview
- Export SVG/PNG download buttons
- Obsidian preprocessor ([[links]], ![[img]], ==highlight==)

frontend/index.html

@@ -61,6 +61,7 @@
         />
         <script src="https://cdnjs.cloudflare.com/ajax/libs/highlight.js/11.9.0/highlight.min.js"></script>
         <script src="https://unpkg.com/[email protected]/dist/umd/lucide.min.js"></script>
+        <script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
         <script type="importmap">
             {
                 "imports": {