fix(GRO-2094): instrument bootstrap with global error + ErrorBoundary

The bundle at /login was executing but the React tree never painted —
no console errors, no fallback UI, just an empty <div id='root'>.
Add three layers of defense so a future failure of this shape is
captured instead of being silently swallowed:

  1. window 'error' and 'unhandledrejection' listeners in main.tsx,
     printing structured context to console.error so Playwright
     sees the failure in the console log even if React unmounts
     the tree.

  2. A top-level <ErrorBoundary> in main.tsx that renders the
     actual exception (name, message, stack) inside the DOM
     instead of leaving <div id='root'> empty. The boundary also
     logs to console.error via componentDidCatch.

  3. New tests for the ErrorBoundary (renders children, surfaces
     thrown errors visibly) and two new UAT_PLAYBOOK test cases
     (TC-WEB-5.1.6 / 5.1.7) that explicitly assert the
     'never-blank-root' invariant on UAT.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/main.tsx

@@ -2,9 +2,41 @@ import { StrictMode } from "react";
 import { createRoot } from "react-dom/client";
 import { BrowserRouter } from "react-router-dom";
 import { App } from "./App.js";
+import { ErrorBoundary } from "./ErrorBoundary.js";
 import { installDevFetchInterceptor } from "./lib/devFetch.js";
 import "./index.css";
 
+// --------------------------------------------------------------------
+// Global error capture (GRO-2094).
+//
+// Symptom: React root stays empty at /login — bundle parses, no console
+// errors, no error boundary fallback. Some failure is being swallowed
+// before it reaches React's commit phase. These listeners make sure any
+// thrown error or unhandled promise rejection is at least visible in
+// the console (and in the Playwright network/console log) instead of
+// vanishing into the void.
+// --------------------------------------------------------------------
+function reportGlobalError(kind: string, payload: unknown): void {
+  // eslint-disable-next-line no-console
+  console.error(`[${kind}]`, payload);
+}
+
+window.addEventListener("error", (event) => {
+  reportGlobalError("window.error", {
+    message: event.message,
+    filename: event.filename,
+    lineno: event.lineno,
+    colno: event.colno,
+    error: event.error,
+  });
+});
+
+window.addEventListener("unhandledrejection", (event) => {
+  reportGlobalError("unhandledrejection", {
+    reason: event.reason,
+  });
+});
+
 installDevFetchInterceptor();
 
 const root = document.getElementById("root");
@@ -12,8 +44,10 @@ if (!root) throw new Error("Root element not found");
 
 createRoot(root).render(
   <StrictMode>
-    <BrowserRouter>
-      <App />
-    </BrowserRouter>
+    <ErrorBoundary>
+      <BrowserRouter>
+        <App />
+      </BrowserRouter>
+    </ErrorBoundary>
   </StrictMode>
 );