fix: Excalidraw loading forever — ReactDOM.createRoot undefined

Root cause: 'import ReactDOM from react-dom' resolves to the root
react-dom package which does NOT export createRoot in React 18.
createRoot lives in react-dom/client.

Changes:
- Changed import to 'react-dom/client' (line 109)
- Added error handling around the render call so loading spinner
  shows an error message instead of spinning forever on failures

frontend/excalidraw-editor.html

@@ -106,7 +106,7 @@
   <!-- Single module script -->
   <script type="module">
     import React from "react";
-    import ReactDOM from "react-dom";
+    import ReactDOM from "react-dom/client";
     import { decompressFromBase64 } from "https://esm.sh/[email protected]";
     import * as ExcalidrawLib from "https://esm.sh/@excalidraw/[email protected]?alias=react:[email protected],react-dom:[email protected]";
 
@@ -246,12 +246,22 @@
             currentTheme = msg.theme || "light";
             setTheme(currentTheme);
 
-            ReactDOM.createRoot(document.getElementById("root")).render(
-              React.createElement(App, {
-                initialData: initialData,
-                theme: currentTheme,
-              })
-            );
+            try {
+              if (typeof ReactDOM.createRoot !== "function") {
+                throw new Error("ReactDOM.createRoot is not available — check react-dom/client import");
+              }
+              ReactDOM.createRoot(document.getElementById("root")).render(
+                React.createElement(App, {
+                  initialData: initialData,
+                  theme: currentTheme,
+                })
+              );
+            } catch (renderErr) {
+              console.error("Excalidraw render failed:", renderErr);
+              loadingEl.textContent = "Error loading Excalidraw: " + renderErr.message;
+              loadingEl.style.color = "#f44336";
+              loadingEl.classList.remove("hidden");
+            }
           }
           break;