refactor: 提取 useToast hook + Toast 组件，消除 4 处重复的通知逻辑

将 state + setTimeout 自动消失逻辑封装为 useToast hook，
Toast UI 统一为组件支持 top/bottom 两种位置，净减约 12 行。

src/hooks/useToast.ts

@@ -0,0 +1,17 @@
+import { useState, useCallback, useRef } from "react";
+
+export function useToast(duration = 2200) {
+  const [message, setMessage] = useState("");
+  const timerRef = useRef<ReturnType<typeof setTimeout>>(undefined);
+
+  const show = useCallback(
+    (msg: string) => {
+      clearTimeout(timerRef.current);
+      setMessage(msg);
+      timerRef.current = setTimeout(() => setMessage(""), duration);
+    },
+    [duration],
+  );
+
+  return { message, show } as const;
+}