Add login gate, calculation history, and AI markdown download.

Co-authored-by: Cursor <cursoragent@cursor.com>

components/auth/auth-provider.tsx

@@ -0,0 +1,80 @@
+"use client";
+
+import {
+  createContext,
+  useCallback,
+  useContext,
+  useEffect,
+  useMemo,
+  useState,
+} from "react";
+
+interface AuthState {
+  loading: boolean;
+  authEnabled: boolean;
+  loggedIn: boolean;
+  username?: string;
+  refresh: () => Promise<void>;
+  logout: () => Promise<void>;
+}
+
+const AuthContext = createContext<AuthState | null>(null);
+
+export function AuthProvider({ children }: { children: React.ReactNode }) {
+  const [loading, setLoading] = useState(true);
+  const [authEnabled, setAuthEnabled] = useState(false);
+  const [loggedIn, setLoggedIn] = useState(true);
+  const [username, setUsername] = useState<string>();
+
+  const refresh = useCallback(async () => {
+    try {
+      const res = await fetch("/api/auth/me", { cache: "no-store" });
+      const data = await res.json();
+      setAuthEnabled(!!data.authEnabled);
+      setLoggedIn(!!data.loggedIn);
+      setUsername(data.username);
+    } catch {
+      setAuthEnabled(false);
+      setLoggedIn(true);
+    } finally {
+      setLoading(false);
+    }
+  }, []);
+
+  useEffect(() => {
+    refresh();
+  }, [refresh]);
+
+  const logout = useCallback(async () => {
+    await fetch("/api/auth/logout", { method: "POST" });
+    await refresh();
+    window.location.href = "/";
+  }, [refresh]);
+
+  const value = useMemo(
+    () => ({ loading, authEnabled, loggedIn, username, refresh, logout }),
+    [loading, authEnabled, loggedIn, username, refresh, logout],
+  );
+
+  return <AuthContext.Provider value={value}>{children}</AuthContext.Provider>;
+}
+
+export function useAuth() {
+  const ctx = useContext(AuthContext);
+  if (!ctx) {
+    throw new Error("useAuth must be used within AuthProvider");
+  }
+  return ctx;
+}
+
+export function useRequireAuthForPath(path: string): boolean {
+  const { authEnabled, loggedIn } = useAuth();
+  if (!authEnabled) {
+    return true;
+  }
+  const protectedPaths = ["/liuyao", "/bazi", "/combined", "/history"];
+  if (!protectedPaths.some((p) => path === p || path.startsWith(`${p}/`))) {
+    return true;
+  }
+  return loggedIn;
+}