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/app.js

@@ -7828,6 +7828,12 @@
     FindInPageManager.init();
     ContextMenuManager.init();
 
+    // Mermaid.js init
+    import('./mermaid-viewer.js').then(function(m) {
+      m.initMermaid();
+      m.setupFocusMode();
+    });
+
     // Check auth status first
     const authOk = await AuthManager.initAuth();