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/components/dev/ComponentShowcase.tsx

@@ -9,11 +9,10 @@
 'use client';
 
 import { useState } from 'react';
-import Link from 'next/link';
 import {
-  Moon, Sun, Mail, User,
+  Mail, User,
   Settings, LogOut, Shield, AlertCircle, Info,
-  Trash2, ArrowLeft
+  Trash2
 } from 'lucide-react';
 import { Button } from '@/components/ui/button';
 import {
@@ -73,41 +72,10 @@ import { Example, ExampleGrid, ExampleSection } from './Example';
  * Component showcase
  */
 export function ComponentShowcase() {
-  const [isDark, setIsDark] = useState(false);
   const [checked, setChecked] = useState(false);
 
-  const toggleTheme = () => {
-    setIsDark(!isDark);
-    document.documentElement.classList.toggle('dark');
-  };
-
   return (
-    <div className="min-h-screen bg-background">
-      {/* Header */}
-      <header className="sticky top-0 z-50 w-full border-b bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/60">
-        <div className="container mx-auto flex h-16 items-center justify-between px-4">
-          <div className="flex items-center gap-4">
-            <Link href="/dev">
-              <Button variant="ghost" size="icon" aria-label="Back to hub">
-                <ArrowLeft className="h-5 w-5" />
-              </Button>
-            </Link>
-            <div>
-              <h1 className="text-xl font-bold">Component Showcase</h1>
-              <p className="text-sm text-muted-foreground">All shadcn/ui components with code</p>
-            </div>
-          </div>
-          <Button
-            variant="outline"
-            size="icon"
-            onClick={toggleTheme}
-            aria-label="Toggle theme"
-          >
-            {isDark ? <Sun className="h-5 w-5" /> : <Moon className="h-5 w-5" />}
-          </Button>
-        </div>
-      </header>
-
+    <div className="bg-background">
       {/* Content */}
       <main className="container mx-auto px-4 py-8">
         <div className="space-y-12">

frontend/src/components/dev/DevLayout.tsx

@@ -0,0 +1,119 @@
+/* istanbul ignore file */
+
+/**
+ * DevLayout Component
+ * Shared layout for all /dev routes with navigation and theme toggle
+ * This file is excluded from coverage as it's a development tool
+ */
+
+'use client';
+
+import Link from 'next/link';
+import { usePathname } from 'next/navigation';
+import { Code2, Palette, LayoutDashboard, Box, FileText, BookOpen, Home } from 'lucide-react';
+import { Button } from '@/components/ui/button';
+import { Badge } from '@/components/ui/badge';
+import { ThemeToggle } from '@/components/theme';
+import { cn } from '@/lib/utils';
+
+interface DevLayoutProps {
+  children: React.ReactNode;
+}
+
+const navItems = [
+  {
+    title: 'Hub',
+    href: '/dev',
+    icon: Home,
+    exact: true,
+  },
+  {
+    title: 'Components',
+    href: '/dev/components',
+    icon: Box,
+  },
+  {
+    title: 'Forms',
+    href: '/dev/forms',
+    icon: FileText,
+  },
+  {
+    title: 'Layouts',
+    href: '/dev/layouts',
+    icon: LayoutDashboard,
+  },
+  {
+    title: 'Spacing',
+    href: '/dev/spacing',
+    icon: Palette,
+  },
+  {
+    title: 'Docs',
+    href: '/dev/docs',
+    icon: BookOpen,
+  },
+];
+
+export function DevLayout({ children }: DevLayoutProps) {
+  const pathname = usePathname();
+
+  const isActive = (href: string, exact?: boolean) => {
+    if (exact) {
+      return pathname === href;
+    }
+    return pathname.startsWith(href);
+  };
+
+  return (
+    <div className="min-h-screen bg-background">
+      {/* Header */}
+      <header className="sticky top-0 z-50 w-full border-b bg-background/95 backdrop-blur supports-[backdrop-filter]:bg-background/60">
+        <div className="container mx-auto px-4">
+          {/* Single Row: Logo + Badge + Navigation + Theme Toggle */}
+          <div className="flex h-14 items-center justify-between gap-6">
+            {/* Left: Logo + Badge */}
+            <div className="flex items-center gap-3 shrink-0">
+              <Code2 className="h-5 w-5 text-primary" />
+              <h1 className="text-base font-semibold">FastNext</h1>
+              <Badge variant="secondary" className="text-xs">
+                Dev
+              </Badge>
+            </div>
+
+            {/* Center: Navigation */}
+            <nav className="flex gap-1 overflow-x-auto flex-1">
+              {navItems.map((item) => {
+                const Icon = item.icon;
+                const active = isActive(item.href, item.exact);
+
+                return (
+                  <Link key={item.href} href={item.href}>
+                    <Button
+                      variant={active ? 'default' : 'ghost'}
+                      size="sm"
+                      className={cn(
+                        'gap-2 whitespace-nowrap',
+                        !active && 'text-muted-foreground hover:text-foreground'
+                      )}
+                    >
+                      <Icon className="h-4 w-4" />
+                      {item.title}
+                    </Button>
+                  </Link>
+                );
+              })}
+            </nav>
+
+            {/* Right: Theme Toggle */}
+            <div className="shrink-0">
+              <ThemeToggle />
+            </div>
+          </div>
+        </div>
+      </header>
+
+      {/* Main Content */}
+      <main>{children}</main>
+    </div>
+  );
+}