fix: HTML code blocks no longer break chat flow

- HTML code blocks now show as syntax-highlighted code with a Preview button
- Added sandboxed iframe HTML preview panel
- Added 'Open in Browser' button for full-screen preview
- Raw HTML from AI responses no longer injected into chat DOM

www/css/styles.css

@@ -431,6 +431,33 @@ a:hover { text-decoration: underline; }
 }
 .copy-btn:hover, .download-btn:hover { background: var(--accent); color: white; }
 
+.html-preview-block { margin: 8px 0; }
+.html-preview-bar {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 6px 12px;
+    background: var(--bg-tertiary);
+    border-radius: 8px 8px 0 0;
+    font-size: 12px;
+    color: var(--text-secondary);
+    border: 1px solid var(--border);
+    border-bottom: none;
+}
+.html-preview-bar + pre { border-top-left-radius: 0; border-top-right-radius: 0; }
+.html-preview-btn {
+    background: linear-gradient(135deg, #6c63ff, #a855f7);
+    border: none;
+    color: white;
+    padding: 4px 14px;
+    border-radius: 4px;
+    font-size: 11px;
+    font-weight: 600;
+    cursor: pointer;
+    transition: opacity var(--transition);
+}
+.html-preview-btn:hover { opacity: 0.85; }
+
 .msg-actions {
     display: flex;
     gap: 6px;