fix: AuthModal 重开时重置表单 + 邀请页加入失败显示错误

- #13: useEffect 监听 open 变化时重置所有表单状态（用户名、密码、错误等）
- #14: handleJoin catch 中捕获错误消息并渲染到页面

src/components/AuthModal.tsx

@@ -1,6 +1,6 @@
 "use client";
 
-import { useState } from "react";
+import { useState, useEffect } from "react";
 import { motion } from "framer-motion";
 import { X, Eye, EyeOff } from "lucide-react";
 import { AVATARS } from "@/lib/avatars";
@@ -36,8 +36,17 @@ export default function AuthModal({ open, onClose, onAuth, defaultTab = "login"
     setAvatar(AVATARS[0].emoji);
     setShowPassword(false);
     setError("");
+    setLoading(false);
   };
 
+  useEffect(() => {
+    if (open) {
+      setTab(defaultTab);
+      resetForm();
+    }
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [open]);
+
   const switchTab = (t: Tab) => {
     setTab(t);
     resetForm();