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/app/blindbox/[code]/page.tsx

@@ -23,7 +23,6 @@ import {
 import confetti from "canvas-confetti";
 import { getCachedProfile, isRegistered } from "@/lib/userId";
 import ShareCardModal from "@/components/ShareCardModal";
-import Toast from "@/components/Toast";
 import { useToast } from "@/hooks/useToast";
 import { BlindboxRoomSkeleton } from "@/components/Skeleton";
 import type { UserProfile } from "@/types";
@@ -815,7 +814,6 @@ export default function BlindboxRoomPage() {
             drawer: revealedIdea.drawnBy ?? undefined,
             roomName: room.name,
           }}
-          onToast={toast.show}
         />
       )}
 
@@ -850,8 +848,6 @@ export default function BlindboxRoomPage() {
         </motion.div>
       )}
 
-      <Toast message={toast.message} position="bottom" />
-
       <div className="h-8 shrink-0" />
     </div>
   );