feat: auth-aware UI and role-based access control (#67)

## Summary

- Add `is_admin` column to users table with Alembic migration and a `require_admin` FastAPI dependency that protects all admin-facing write endpoints (games, pokemon, evolutions, bosses, routes CRUD)
- Expose admin status to frontend via user API and update AuthContext to fetch/store `isAdmin` after login
- Make navigation menu auth-aware (different links for logged-out, logged-in, and admin users) and protect frontend routes with `ProtectedRoute` and `AdminRoute` components, preserving deep-linking through redirects
- Fix test reliability: `drop_all` before `create_all` to clear stale PostgreSQL enums from interrupted test runs
- Fix test auth: add `admin_client` fixture and use valid UUID for mock user so tests pass with new admin-protected endpoints

## Test plan

- [x] All 252 backend tests pass
- [ ] Verify non-admin users cannot access admin write endpoints (games, pokemon, evolutions, bosses CRUD)
- [ ] Verify admin users can access admin endpoints normally
- [ ] Verify navigation shows correct links for logged-out, logged-in, and admin states
- [ ] Verify `/admin/*` routes redirect non-admin users with a toast
- [ ] Verify `/runs/new` and `/genlockes/new` redirect unauthenticated users to login, then back after auth

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Reviewed-on: #67
Co-authored-by: Julian Tabel <juliantabel.jt@gmail.com>
Co-committed-by: Julian Tabel <juliantabel.jt@gmail.com>

frontend/src/components/Layout.tsx

@@ -1,16 +1,8 @@
-import { useState } from 'react'
+import { useState, useMemo } from 'react'
 import { Link, Outlet, useLocation } from 'react-router-dom'
 import { useTheme } from '../hooks/useTheme'
 import { useAuth } from '../contexts/AuthContext'
 
-const navLinks = [
-  { to: '/runs/new', label: 'New Run' },
-  { to: '/runs', label: 'My Runs' },
-  { to: '/genlockes', label: 'Genlockes' },
-  { to: '/stats', label: 'Stats' },
-  { to: '/admin', label: 'Admin' },
-]
-
 function NavLink({
   to,
   active,
@@ -136,9 +128,34 @@ function UserMenu({ onAction }: { onAction?: () => void }) {
 export function Layout() {
   const [menuOpen, setMenuOpen] = useState(false)
   const location = useLocation()
+  const { user, isAdmin } = useAuth()
+
+  const navLinks = useMemo(() => {
+    if (!user) {
+      // Logged out: Home, Runs, Genlockes, Stats
+      return [
+        { to: '/', label: 'Home' },
+        { to: '/runs', label: 'Runs' },
+        { to: '/genlockes', label: 'Genlockes' },
+        { to: '/stats', label: 'Stats' },
+      ]
+    }
+    // Logged in: New Run, My Runs, Genlockes, Stats
+    const links = [
+      { to: '/runs/new', label: 'New Run' },
+      { to: '/runs', label: 'My Runs' },
+      { to: '/genlockes', label: 'Genlockes' },
+      { to: '/stats', label: 'Stats' },
+    ]
+    // Admin gets Admin link
+    if (isAdmin) {
+      links.push({ to: '/admin', label: 'Admin' })
+    }
+    return links
+  }, [user, isAdmin])
 
   function isActive(to: string) {
-    if (to === '/runs/new') return location.pathname === '/runs/new'
+    if (to === '/' || to === '/runs/new') return location.pathname === to
     return location.pathname.startsWith(to)
   }