fix(auth): gate SessionProvider + CloudSyncButton behind NEXT_PUBLIC_ENABLE_CLOUD_SYNC=1

HF Space crashed because NextAuth requires NEXTAUTH_SECRET in production
and SessionProvider auto-fetches /api/auth/session on mount. With
cloud-sync env vars unset, that 500s and brings down the space.

Now: cloud-sync UI + provider only render when explicitly enabled. Set
NEXT_PUBLIC_ENABLE_CLOUD_SYNC=1 (plus the 6 backend env vars) to re-enable.

components/auth-provider.tsx

@@ -2,6 +2,11 @@
 import { SessionProvider } from "next-auth/react"
 import { ReactNode } from "react"
 
+const ENABLED = process.env.NEXT_PUBLIC_ENABLE_CLOUD_SYNC === "1"
+
 export function AuthProvider({ children }: { children: ReactNode }) {
+  // Tắt SessionProvider nếu cloud-sync chưa được cấu hình.
+  // Tránh /api/auth/session bị gọi → 500 vì thiếu NEXTAUTH_SECRET.
+  if (!ENABLED) return <>{children}</>
   return <SessionProvider>{children}</SessionProvider>
 }

components/cloud-sync-button.tsx

@@ -23,6 +23,13 @@ function formatRelative(ts: number | null): string {
 }
 
 export function CloudSyncButton() {
+  // Tắt hoàn toàn nếu cloud-sync chưa được cấu hình (env NEXT_PUBLIC_ENABLE_CLOUD_SYNC).
+  // useSession sẽ throw nếu không có SessionProvider phía trên, nên gate ngay tại đây.
+  if (process.env.NEXT_PUBLIC_ENABLE_CLOUD_SYNC !== "1") return null
+  return <CloudSyncButtonInner />
+}
+
+function CloudSyncButtonInner() {
   const { data: session, status: authStatus } = useSession()
   const { status, lastSyncAt, error, pullAll, pushAll } = useCloudSync()
   const [open, setOpen] = useState(false)