fix(web): prevent click event from being stored as document title

The sidebar "+ Document" button was passing the click event to
onNewDocument, which stored a SyntheticEvent object as the title,
causing React error #31 when rendering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

calcpad-web/src/components/Sidebar.tsx

@@ -432,7 +432,7 @@ export function Sidebar({
 
         {/* Footer */}
         <div className="sidebar-footer">
-          <button className="sidebar-footer-btn" onClick={onNewDocument}>+ Document</button>
+          <button className="sidebar-footer-btn" onClick={() => onNewDocument()}>+ Document</button>
           <button className="sidebar-footer-btn" onClick={onNewFolder}>+ Folder</button>
         </div>