feat(frontend): implement center auth session flow with route guard

src/auth/auth-context.ts

@@ -0,0 +1,18 @@
+import { createContext, useContext } from 'react'
+import type { AuthSession } from '../lib/auth-storage'
+
+export type AuthContextValue = {
+  session: AuthSession | null
+  isAuthed: boolean
+  login: (email: string, password: string) => Promise<void>
+  logout: () => Promise<void>
+  ensureFreshToken: () => Promise<string | null>
+}
+
+export const AuthContext = createContext<AuthContextValue | null>(null)
+
+export function useAuth() {
+  const ctx = useContext(AuthContext)
+  if (!ctx) throw new Error('useAuth must be used inside <AuthProvider>')
+  return ctx
+}