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/contexts/AuthContext.tsx

@@ -1,11 +1,20 @@
 import { createContext, useContext, useEffect, useState, useCallback, useMemo } from 'react'
 import type { User, Session, AuthError } from '@supabase/supabase-js'
 import { supabase } from '../lib/supabase'
+import { api } from '../api/client'
+
+interface UserProfile {
+  id: string
+  email: string
+  displayName: string | null
+  isAdmin: boolean
+}
 
 interface AuthState {
   user: User | null
   session: Session | null
   loading: boolean
+  isAdmin: boolean
 }
 
 interface AuthContextValue extends AuthState {
@@ -18,22 +27,35 @@ interface AuthContextValue extends AuthState {
 
 const AuthContext = createContext<AuthContextValue | null>(null)
 
+async function syncUserProfile(session: Session | null): Promise<boolean> {
+  if (!session) return false
+  try {
+    const profile = await api.post<UserProfile>('/users/me', {})
+    return profile.isAdmin
+  } catch {
+    return false
+  }
+}
+
 export function AuthProvider({ children }: { children: React.ReactNode }) {
   const [state, setState] = useState<AuthState>({
     user: null,
     session: null,
     loading: true,
+    isAdmin: false,
   })
 
   useEffect(() => {
-    supabase.auth.getSession().then(({ data: { session } }) => {
-      setState({ user: session?.user ?? null, session, loading: false })
+    supabase.auth.getSession().then(async ({ data: { session } }) => {
+      const isAdmin = await syncUserProfile(session)
+      setState({ user: session?.user ?? null, session, loading: false, isAdmin })
     })
 
     const {
       data: { subscription },
-    } = supabase.auth.onAuthStateChange((_event, session) => {
-      setState({ user: session?.user ?? null, session, loading: false })
+    } = supabase.auth.onAuthStateChange(async (_event, session) => {
+      const isAdmin = await syncUserProfile(session)
+      setState({ user: session?.user ?? null, session, loading: false, isAdmin })
     })
 
     return () => subscription.unsubscribe()