refactor: Toast 升级为全局 Context，消除 onToast prop 透传

将 useToast 从独立 state hook 改为 Context-based，在 layout 中
挂载 ToastProvider 全局渲染 Toast。QrInviteModal、RoomManageModal、
ShareCardModal 不再需要 onToast prop，直接 useToast() 调用即可。
父组件 TopNav、MatchResult、profile、blindbox 移除了本地 Toast
渲染和 onToast 传递逻辑。

src/components/ToastProvider.tsx

@@ -0,0 +1,29 @@
+"use client";
+
+import { useState, useCallback, useRef } from "react";
+import { ToastContext, type ToastPosition } from "@/hooks/useToast";
+import Toast from "./Toast";
+
+export default function ToastProvider({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  const [message, setMessage] = useState("");
+  const [position, setPosition] = useState<ToastPosition>("top");
+  const timerRef = useRef<ReturnType<typeof setTimeout>>(undefined);
+
+  const show = useCallback((msg: string, pos: ToastPosition = "top") => {
+    clearTimeout(timerRef.current);
+    setMessage(msg);
+    setPosition(pos);
+    timerRef.current = setTimeout(() => setMessage(""), 2200);
+  }, []);
+
+  return (
+    <ToastContext.Provider value={{ show }}>
+      {children}
+      <Toast message={message} position={position} />
+    </ToastContext.Provider>
+  );
+}