fix: resolve black screen caused by React hooks violation in WaterfallModal

Move useMemo before early return in WaterfallModal to comply with Rules of Hooks.
Wrap App with ErrorBoundary so future render crashes show an error panel instead of a silent black screen.

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

index.tsx

@@ -1,6 +1,7 @@
 import React from 'react';
 import ReactDOM from 'react-dom/client';
 import App from './App';
+import ErrorBoundary from './components/ErrorBoundary';
 
 const rootElement = document.getElementById('root');
 if (!rootElement) {
@@ -10,6 +11,8 @@ if (!rootElement) {
 const root = ReactDOM.createRoot(rootElement);
 root.render(
   <React.StrictMode>
-    <App />
+    <ErrorBoundary>
+      <App />
+    </ErrorBoundary>
   </React.StrictMode>
 );