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

src/auth/AuthContext.tsx

@@ -0,0 +1,52 @@
+import { useMemo, useState } from 'react'
+import type { PropsWithChildren } from 'react'
+import { clearAuthSession, getAuthSession, isAccessTokenStale, setAuthSession } from '../lib/auth-storage'
+import type { AuthSession } from '../lib/auth-storage'
+import { loginCenter, logoutCenter, refreshCenter } from '../lib/center-auth-client'
+import { AuthContext } from './auth-context'
+import type { AuthContextValue } from './auth-context'
+
+export function AuthProvider({ children }: PropsWithChildren) {
+  const [session, setSession] = useState<AuthSession | null>(getAuthSession())
+
+  const value = useMemo<AuthContextValue>(
+    () => ({
+      session,
+      isAuthed: !!session,
+      login: async (email: string, password: string) => {
+        const next = await loginCenter({ email, password })
+        setAuthSession(next)
+        setSession(next)
+      },
+      logout: async () => {
+        if (session?.refreshToken) {
+          try {
+            await logoutCenter(session.refreshToken)
+          } catch {
+            // noop
+          }
+        }
+        clearAuthSession()
+        setSession(null)
+      },
+      ensureFreshToken: async () => {
+        if (!session) return null
+        if (!isAccessTokenStale(session.accessToken)) return session.accessToken
+
+        const refreshed = await refreshCenter(session.refreshToken)
+        const next: AuthSession = {
+          ...session,
+          accessToken: refreshed.accessToken,
+          refreshToken: refreshed.refreshToken,
+          tokenType: refreshed.tokenType,
+        }
+        setAuthSession(next)
+        setSession(next)
+        return next.accessToken
+      },
+    }),
+    [session],
+  )
+
+  return <AuthContext.Provider value={value}>{children}</AuthContext.Provider>
+}