Revert Zustand persist middleware approach and restore AuthInitializer

- Remove persist middleware from authStore (causing hooks timing issues)
- Restore original AuthInitializer component pattern
- Keep good Phase 3 optimizations:
  - Theme FOUC fix (inline script)
  - React Query refetchOnWindowFocus disabled
  - Code splitting for dev/auth components
  - Shared form components (FormField, useFormError)
  - Store location in lib/stores

frontend/src/app/providers.tsx

@@ -4,6 +4,7 @@ import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
 import { ReactQueryDevtools } from '@tanstack/react-query-devtools';
 import { useState } from 'react';
 import { ThemeProvider } from '@/components/theme';
+import { AuthInitializer } from '@/components/auth';
 
 export function Providers({ children }: { children: React.ReactNode }) {
   const [queryClient] = useState(
@@ -26,7 +27,7 @@ export function Providers({ children }: { children: React.ReactNode }) {
   return (
     <ThemeProvider>
       <QueryClientProvider client={queryClient}>
-        {/* AuthInitializer removed - Zustand persist middleware handles auto-hydration */}
+        <AuthInitializer />
         {children}
         <ReactQueryDevtools initialIsOpen={false} />
       </QueryClientProvider>