Refactor auth hooks and add database existence check during migrations

- Consolidated `useAuthStore` into the unified `useAuth` hook for cleaner imports and consistency across frontend components.
- Enhanced database management in Alembic migrations by introducing `ensure_database_exists` to automatically create the database if missing.

frontend/src/components/auth/AuthGuard.tsx

@@ -8,7 +8,7 @@
 
 import { useEffect, useState } from 'react';
 import { useRouter, usePathname } from 'next/navigation';
-import { useAuthStore } from '@/lib/stores/authStore';
+import { useAuth } from '@/lib/stores';
 import { useMe } from '@/lib/api/hooks/useAuth';
 import { AuthLoadingSkeleton } from '@/components/layout';
 import config from '@/config/app.config';
@@ -50,7 +50,7 @@ interface AuthGuardProps {
 export function AuthGuard({ children, requireAdmin = false, fallback }: AuthGuardProps) {
   const router = useRouter();
   const pathname = usePathname();
-  const { isAuthenticated, isLoading: authLoading, user } = useAuthStore();
+  const { isAuthenticated, isLoading: authLoading, user } = useAuth();
 
   // Delayed loading state - only show skeleton after 150ms to avoid flicker on fast loads
   const [showLoading, setShowLoading] = useState(false);

frontend/src/components/layout/Header.tsx

@@ -8,7 +8,7 @@
 
 import Link from 'next/link';
 import { usePathname } from 'next/navigation';
-import { useAuthStore } from '@/lib/stores/authStore';
+import { useAuth } from '@/lib/stores';
 import { useLogout } from '@/lib/api/hooks/useAuth';
 import {
   DropdownMenu,
@@ -67,7 +67,7 @@ function NavLink({
 }
 
 export function Header() {
-  const { user } = useAuthStore();
+  const { user } = useAuth();
   const { mutate: logout, isPending: isLoggingOut } = useLogout();
 
   const handleLogout = () => {