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/index.html

@@ -132,6 +132,19 @@
                 </div>
             </div>
 
+            <div id="html-preview" class="file-viewer" style="display:none;z-index:2500;">
+                <div class="file-viewer-header">
+                    <div class="file-viewer-title">
+                        <span id="html-preview-title">HTML Preview</span>
+                    </div>
+                    <div class="file-viewer-actions">
+                        <button id="html-preview-newtab" class="fv-btn">Open in Browser</button>
+                        <button id="html-preview-close" class="icon-btn" aria-label="Close preview">&times;</button>
+                    </div>
+                </div>
+                <iframe id="html-preview-frame" sandbox="allow-scripts allow-same-origin" style="flex:1;width:100%;border:none;background:white;border-radius:0 0 12px 12px;"></iframe>
+            </div>
+
             <div id="messages" class="messages"></div>
 
             <div id="terminal-panel" class="terminal-panel">