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/__tests__/ErrorBoundary.test.tsx

@@ -0,0 +1,54 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { render, screen, cleanup } from "@testing-library/react";
+import { ErrorBoundary } from "../ErrorBoundary";
+
+function ThrowingChild(): never {
+  throw new Error("synthetic render-time failure for GRO-2094");
+}
+
+function GoodChild() {
+  return <div data-testid="good-child">ok</div>;
+}
+
+describe("ErrorBoundary (GRO-2094)", () => {
+  let errorSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach(() => {
+    // React 18+ logs caught render errors to console.error via React's own
+    // instrumentation; suppress it so test output is clean but capture it
+    // for an assertion below.
+    errorSpy = vi.spyOn(console, "error").mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    errorSpy.mockRestore();
+    cleanup();
+  });
+
+  it("renders children when nothing throws", () => {
+    render(
+      <ErrorBoundary>
+        <GoodChild />
+      </ErrorBoundary>
+    );
+    expect(screen.getByTestId("good-child")).toBeInTheDocument();
+    expect(screen.queryByTestId("error-boundary")).not.toBeInTheDocument();
+  });
+
+  it("renders the error visibly when a child throws during render", () => {
+    render(
+      <ErrorBoundary>
+        <ThrowingChild />
+      </ErrorBoundary>
+    );
+
+    const fallback = screen.getByTestId("error-boundary");
+    expect(fallback).toBeInTheDocument();
+    const message = screen.getByTestId("error-boundary-message");
+    // The actual exception is shown — no more silent blank root.
+    expect(message.textContent).toContain("synthetic render-time failure for GRO-2094");
+    // The boundary also calls console.error so it shows up in the Playwright
+    // console log even if the DOM-rendered fallback is somehow missed.
+    expect(errorSpy).toHaveBeenCalled();
+  });
+});