refactor: 提取 useShare hook，统一分享和剪贴板逻辑

新增 useShare（Web Share API + clipboard fallback），
消除 QrInviteModal、BlindboxRoomPage、ShareCardModal 三处重复的分享代码。

src/hooks/useShare.ts

@@ -0,0 +1,39 @@
+"use client";
+
+import { useCallback } from "react";
+import { useToast } from "@/hooks/useToast";
+
+export function useShare() {
+  const toast = useToast();
+
+  const copyToClipboard = useCallback(
+    async (text: string, successMsg = "已复制") => {
+      try {
+        await navigator.clipboard.writeText(text);
+        toast.show(successMsg);
+      } catch {
+        toast.show("复制失败，请手动复制");
+      }
+    },
+    [toast],
+  );
+
+  /** Try the native Web Share API; calls `fallback` if unavailable. Returns true if native share was triggered. */
+  const share = useCallback(
+    async (data: ShareData, fallback?: () => void): Promise<boolean> => {
+      try {
+        if (navigator.share && navigator.canShare?.(data)) {
+          await navigator.share(data);
+          return true;
+        }
+      } catch (e) {
+        if (e instanceof Error && e.name === "AbortError") return true;
+      }
+      fallback?.();
+      return false;
+    },
+    [],
+  );
+
+  return { share, copyToClipboard };
+}