Enable <StrictMode> + make App.tsx renderer init idempotent

App.tsx's prepare() useEffect ran four .init() calls
(SessionDataManager, RoomSessionManager, RoomEngine, Communication)
without any guard, plus an immediate heartbeat ping and a legacy
authentication track. Under StrictMode dev double-invoke, those
fire twice — risking duplicate session/communication state.

- Gate the four .init() chain behind gameInitPromiseRef: both the
  first and the simulated second invocation await the same promise.
- Gate the legacy track + immediate heartbeat behind bootstrapDoneRef.
- Heartbeat and remember-rotate intervals were already idempotent
  (clearInterval before setInterval); ticker registration was already
  guarded by tickersStartedRef; renderer/warmup were already gated by
  rendererPromiseRef/warmupPromiseRef. No change needed there.

Wrap <App /> in <StrictMode> in src/index.tsx now that the renderer
init path is double-invoke safe.

https://claude.ai/code/session_01GrR87LAqnAEyKG2ZbmQt5Q

src/index.tsx

@@ -1,4 +1,4 @@
-import { Suspense } from 'react';
+import { StrictMode, Suspense } from 'react';
 import { createRoot } from 'react-dom/client';
 import { ErrorBoundary } from 'react-error-boundary';
 import { App } from './App';
@@ -44,15 +44,17 @@ import './css/toolbar/ToolBar.css';
 import './css/widgets/FurnitureWidgets.css';
 
 createRoot(document.getElementById('root')).render(
-    <ErrorBoundary
-        fallbackRender={ ({ error }) => (
-            <LoadingView
-                isError={ true }
-                message={ `Something went wrong.\n${ (error as Error)?.message ?? 'Unknown error' }` }
-                homeUrl={ window.location.origin + '/' } />
-        ) }>
-        <Suspense fallback={ <LoadingView message="Loading…" /> }>
-            <App />
-        </Suspense>
-    </ErrorBoundary>
+    <StrictMode>
+        <ErrorBoundary
+            fallbackRender={ ({ error }) => (
+                <LoadingView
+                    isError={ true }
+                    message={ `Something went wrong.\n${ (error as Error)?.message ?? 'Unknown error' }` }
+                    homeUrl={ window.location.origin + '/' } />
+            ) }>
+            <Suspense fallback={ <LoadingView message="Loading…" /> }>
+                <App />
+            </Suspense>
+        </ErrorBoundary>
+    </StrictMode>
 );