feat: 撤回滑动功能，按钮移至进度条旁

- 新增 POST /api/room/[id]/undo 端点，撤回 like 和 swipeCount
- 进度条右侧显示"↩ 撤回"按钮，可一直撤回到第一张
- ActionButtons 恢复干净的两按钮布局，避免误触

src/app/api/room/[id]/undo/route.ts

@@ -0,0 +1,57 @@
+import { NextResponse } from "next/server";
+import { atomicUpdateRoom } from "@/lib/store";
+
+export async function POST(
+  req: Request,
+  { params }: { params: Promise<{ id: string }> },
+) {
+  const { id } = await params;
+
+  try {
+    const { userId, restaurantId } = await req.json();
+
+    if (!userId || restaurantId == null) {
+      return NextResponse.json(
+        { error: "userId and restaurantId are required" },
+        { status: 400 },
+      );
+    }
+
+    const rid = String(restaurantId);
+
+    const updated = await atomicUpdateRoom(id, (data) => {
+      if (data.likes[rid]) {
+        data.likes[rid] = data.likes[rid].filter((u) => u !== userId);
+        if (data.likes[rid].length === 0) {
+          delete data.likes[rid];
+        }
+      }
+
+      if (data.match === rid) {
+        data.match = null;
+      }
+
+      const count = data.swipeCounts[userId] ?? 0;
+      if (count > 0) {
+        data.swipeCounts[userId] = count - 1;
+      }
+
+      return data;
+    });
+
+    if (!updated) {
+      return NextResponse.json(
+        { error: "房间不存在或已过期" },
+        { status: 404 },
+      );
+    }
+
+    return NextResponse.json({ ok: true });
+  } catch (e) {
+    console.error("Failed to undo swipe:", e);
+    return NextResponse.json(
+      { error: "撤回失败" },
+      { status: 500 },
+    );
+  }
+}