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/middleware.ts

@@ -0,0 +1,22 @@
+import { NextResponse } from 'next/server';
+import type { NextRequest } from 'next/server';
+
+export function middleware(request: NextRequest) {
+  const { pathname } = request.nextUrl;
+
+  // Block access to /dev routes in production
+  if (pathname.startsWith('/dev')) {
+    const isProduction = process.env.NODE_ENV === 'production';
+
+    if (isProduction) {
+      // Return 404 in production
+      return new NextResponse(null, { status: 404 });
+    }
+  }
+
+  return NextResponse.next();
+}
+
+export const config = {
+  matcher: '/dev/:path*',
+};