feat: Mermaid + highlight.js rendering in preview pane

Added CDN scripts: mermaid@11, highlight.js@11 with atom-one-dark theme. After server-rendered HTML is injected, processPreviewBlocks() renders Mermaid diagrams via mermaid.render() and highlights code blocks via hljs.highlightElement(). Preview now matches ObsiGate viewer fidelity.

frontend/editor-poc.html

@@ -342,8 +342,11 @@ body { font-family: var(--sans); background: var(--bg); color: var(--text); heig
 <!-- Toast -->
 <div class="toast" id="toast"></div>
 
-<!-- marked.js CDN for markdown rendering -->
+<!-- marked.js + highlight.js + Mermaid CDN for preview rendering -->
 <script src="https://cdn.jsdelivr.net/npm/marked/marked.min.js"></script>
+<script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
+<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/highlight.js@11/styles/atom-one-dark.min.css">
+<script src="https://cdn.jsdelivr.net/npm/highlight.js@11/lib/highlight.min.js"></script>
 
 <script>
 (function() {
@@ -640,6 +643,7 @@ body { font-family: var(--sans); background: var(--bg); color: var(--text); heig
       return r.json();
     }).then(function(data) {
       previewContent.innerHTML = data.html || '<div class="preview-loading">Empty document</div>';
+      processPreviewBlocks();
       _previewLoading = false;
     }).catch(function(e) {
       previewContent.innerHTML = '<div class="preview-loading">Render error: ' + e.message + '</div>';
@@ -1161,11 +1165,42 @@ body { font-family: var(--sans); background: var(--bg); color: var(--text); heig
   document.getElementById('poc-title').addEventListener('input', function() { markDirty(); });
 
   // ---- Init ----
+  mermaid.initialize({ startOnLoad: false, theme: 'dark', securityLevel: 'loose' });
   initSlashItems();
   loadFile();
   checkAIStatus();
   updateStats();
 
+  // ---- Process preview blocks (Mermaid + highlight.js) ----
+  function processPreviewBlocks() {
+    // Render Mermaid diagrams
+    var mermaidBlocks = previewContent.querySelectorAll('pre code.language-mermaid');
+    mermaidBlocks.forEach(function(block) {
+      var code = block.textContent;
+      var id = 'mermaid-prev-' + Math.random().toString(36).substr(2, 8);
+      try {
+        mermaid.render(id, code).then(function(result) {
+          var container = document.createElement('div');
+          container.className = 'mermaid-rendered';
+          container.innerHTML = result.svg;
+          block.parentElement.replaceWith(container);
+        }).catch(function() {
+          // Mermaid render failed, leave code block as-is
+        });
+      } catch(e) {
+        // ignore render errors
+      }
+    });
+    // Syntax highlight code blocks
+    if (typeof hljs !== 'undefined') {
+      previewContent.querySelectorAll('pre code').forEach(function(block) {
+        if (!block.classList.contains('language-mermaid')) {
+          hljs.highlightElement(block);
+        }
+      });
+    }
+  }
+
   console.log('ObsiGate Editor v1 ready');
   console.log('  File: ' + (fileVault ? fileVault + '/' + filePath : 'demo mode'));
   console.log('  AI: checking...');