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

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

lib/auth/config.ts

@@ -0,0 +1,41 @@
+export const SESSION_COOKIE = "zhimingge_session";
+export const SESSION_MAX_AGE_SEC = 60 * 60 * 24 * 7;
+
+export function isAuthEnabled(): boolean {
+  return !!(
+    process.env.AUTH_USERNAME?.trim() &&
+    process.env.AUTH_PASSWORD?.trim() &&
+    process.env.AUTH_SESSION_SECRET?.trim()
+  );
+}
+
+export function getAuthUsername(): string {
+  const username = process.env.AUTH_USERNAME?.trim();
+  if (!username) {
+    throw new Error("未配置 AUTH_USERNAME");
+  }
+  return username;
+}
+
+export function getAuthPassword(): string {
+  const password = process.env.AUTH_PASSWORD?.trim();
+  if (!password) {
+    throw new Error("未配置 AUTH_PASSWORD");
+  }
+  return password;
+}
+
+export function getAuthSessionSecret(): string {
+  const secret = process.env.AUTH_SESSION_SECRET?.trim();
+  if (!secret) {
+    throw new Error("未配置 AUTH_SESSION_SECRET");
+  }
+  return secret;
+}
+
+export function verifyCredentials(username: string, password: string): boolean {
+  if (!isAuthEnabled()) {
+    return true;
+  }
+  return username === getAuthUsername() && password === getAuthPassword();
+}