fix: add onerror handler for lazy-loaded WebGL addon script

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/web/public/app.js

@@ -666,6 +666,7 @@ class CodemanApp {
         const wglScript = document.createElement('script');
         wglScript.src = 'vendor/xterm-addon-webgl.min.js';
         wglScript.onload = () => this._initWebGL();
+        wglScript.onerror = () => console.warn('[CRASH-DIAG] Failed to load WebGL addon — using canvas renderer');
         document.head.appendChild(wglScript);
       }
     }