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/hooks/useToast.ts

@@ -1,17 +1,15 @@
-import { useState, useCallback, useRef } from "react";
+import { createContext, useContext } from "react";
 
-export function useToast(duration = 2200) {
-  const [message, setMessage] = useState("");
-  const timerRef = useRef<ReturnType<typeof setTimeout>>(undefined);
+export type ToastPosition = "top" | "bottom";
 
-  const show = useCallback(
-    (msg: string) => {
-      clearTimeout(timerRef.current);
-      setMessage(msg);
-      timerRef.current = setTimeout(() => setMessage(""), duration);
-    },
-    [duration],
-  );
-
-  return { message, show } as const;
+export interface ToastContextValue {
+  show: (msg: string, position?: ToastPosition) => void;
+}
+
+export const ToastContext = createContext<ToastContextValue | null>(null);
+
+export function useToast() {
+  const ctx = useContext(ToastContext);
+  if (!ctx) throw new Error("useToast must be used within ToastProvider");
+  return ctx;
 }