fix(excalidraw): use import map to force React 18.3.1 for ALL modules including Excalidraw internals

frontend/excalidraw-diag3.html

@@ -1,6 +1,9 @@
 <!DOCTYPE html>
 <html>
-<head><meta charset="UTF-8"><title>Excalidraw Error Capture</title>
+<head><meta charset="UTF-8">
+  <meta http-equiv="Cache-Control" content="no-cache, no-store, must-revalidate">
+  <meta http-equiv="Pragma" content="no-cache">
+  <meta http-equiv="Expires" content="0"><title>Excalidraw Error Capture</title>
 <style>
   body { font-family:monospace; background:#1e1e1e; color:#ddd; padding:20px; }
   .pass { color:#4caf50; } .fail { color:#f44336; }
@@ -14,8 +17,8 @@
 <div id="app"></div>
 
 <script type="module">
-import React from "https://esm.sh/react@18";
-import ReactDOM from "https://esm.sh/react-dom@18/client";
+import React from "https://esm.sh/react@18.3.1";
+import ReactDOM from "https://esm.sh/react-dom@18.3.1/client";
 const log = document.getElementById('log');
 function add(msg, cls='') { log.innerHTML += `<span class="${cls}">${msg}</span>\n`; }