feat: expose admin status to frontend via user API

Add is_admin field to UserResponse schema and update AuthContext to
fetch user profile after login, storing and exposing isAdmin boolean.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.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()