feat: preview uses ObsiGate server-side rendering (Mermaid, highlight, wikilinks)

Preview now saves content via API then fetches rendered HTML from GET /api/file/{vault}?path=... instead of client-side marked.parse(). This enables Mermaid diagrams, syntax highlighting, wikilink resolution, and frontmatter cards. Demo mode still uses marked.js fallback. Debounce increased to 500ms. Loading spinner during render.

frontend/editor-poc.html

@@ -604,21 +604,47 @@ body { font-family: var(--sans); background: var(--bg); color: var(--text); heig
     });
   }
 
-  // ---- Markdown Preview ----
+  // ---- Markdown Preview (server-side rendering via ObsiGate API) ----
   var previewTimer = null;
+  var _previewLoading = false;
   function schedulePreview() {
     if (!previewVisible) return;
     clearTimeout(previewTimer);
-    previewTimer = setTimeout(renderPreview, 300);
+    previewTimer = setTimeout(renderPreview, 500);
   }
 
   function renderPreview() {
     if (!previewVisible) return;
-    try {
-      previewContent.innerHTML = marked.parse(val()) || '<div class="preview-loading">Empty document</div>';
-    } catch(e) {
-      previewContent.innerHTML = '<div class="preview-loading">Preview error</div>';
+    if (_previewLoading) return;
+    // Demo mode: fallback to client-side marked
+    if (!fileVault || !filePath) {
+      try {
+        previewContent.innerHTML = marked.parse(val()) || '<div class="preview-loading">Empty document</div>';
+      } catch(e) {
+        previewContent.innerHTML = '<div class="preview-loading">Preview error</div>';
+      }
+      return;
     }
+    // Save first, then fetch server-rendered HTML
+    _previewLoading = true;
+    previewContent.innerHTML = '<div class="preview-loading"><span class="spin"></span> Rendering...</div>';
+    var content = val();
+    fetch('/api/file/' + encodeURIComponent(fileVault) + '/save?path=' + encodeURIComponent(filePath) + '&backup=false', {
+      method: 'PUT', headers: { 'Content-Type': 'application/json' },
+      body: JSON.stringify({ content: content })
+    }).then(function(r) {
+      if (!r.ok) throw new Error('Save failed');
+      return fetch('/api/file/' + encodeURIComponent(fileVault) + '?path=' + encodeURIComponent(filePath));
+    }).then(function(r) {
+      if (!r.ok) throw new Error('Render failed');
+      return r.json();
+    }).then(function(data) {
+      previewContent.innerHTML = data.html || '<div class="preview-loading">Empty document</div>';
+      _previewLoading = false;
+    }).catch(function(e) {
+      previewContent.innerHTML = '<div class="preview-loading">Render error: ' + e.message + '</div>';
+      _previewLoading = false;
+    });
   }
 
   function togglePreview() {
@@ -627,7 +653,7 @@ body { font-family: var(--sans); background: var(--bg); color: var(--text); heig
     resizeHandle.style.display = previewVisible ? '' : 'none';
     btnPreview.style.color = previewVisible ? 'var(--accent)' : '';
     btnPreview.style.background = previewVisible ? 'var(--accent-bg)' : '';
-    if (previewVisible) renderPreview();
+    if (previewVisible) { renderPreview(); }
   }
 
   // ---- Sync scroll between editor and preview ----