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 },
+    );
+  }
+}

src/components/SwipeDeck.tsx

@@ -7,7 +7,7 @@ import ActionButtons from "./ActionButtons";
 import MatchResult from "./MatchResult";
 import SwipeGuide from "./SwipeGuide";
 import { Restaurant, SwipeDirection, MatchType } from "@/types";
-import { Heart } from "lucide-react";
+import { Heart, Undo2 } from "lucide-react";
 
 interface SwipeDeckProps {
   restaurants: Restaurant[];
@@ -38,6 +38,7 @@ export default function SwipeDeck({
   const [resetting, setResetting] = useState(false);
   const [bubble, setBubble] = useState("");
   const [guideVisible, setGuideVisible] = useState(true);
+  const [swipeHistory, setSwipeHistory] = useState<string[]>([]);
   const swipeFnRef = useRef<((direction: SwipeDirection) => void) | null>(null);
   const swipingRef = useRef(false);
   const prevLikeCounts = useRef<Record<string, number>>({});
@@ -104,6 +105,7 @@ export default function SwipeDeck({
       const action = direction === "right" ? "like" : "nope";
       sendSwipe(current.id, action);
 
+      setSwipeHistory((h) => [...h, current.id]);
       const nextIndex = currentIndex + 1;
       setCurrentIndex(nextIndex);
       swipeFnRef.current = null;
@@ -122,6 +124,27 @@ export default function SwipeDeck({
     [],
   );
 
+  const handleUndo = useCallback(async () => {
+    if (swipeHistory.length === 0 || currentIndex === 0) return;
+
+    const lastRid = swipeHistory[swipeHistory.length - 1];
+
+    try {
+      await fetch(`/api/room/${roomId}/undo`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ userId, restaurantId: lastRid }),
+      });
+    } catch {
+      // Best-effort
+    }
+
+    setSwipeHistory((h) => h.slice(0, -1));
+    setCurrentIndex((i) => i - 1);
+    setLocalMatchId(null);
+    swipeFnRef.current = null;
+  }, [swipeHistory, currentIndex, roomId, userId]);
+
   const handleReset = useCallback(async () => {
     setResetting(true);
     try {
@@ -129,6 +152,7 @@ export default function SwipeDeck({
       setCurrentIndex(0);
       setShowMatch(false);
       setLocalMatchId(null);
+      setSwipeHistory([]);
       prevLikeCounts.current = {};
     } finally {
       setResetting(false);
@@ -147,8 +171,8 @@ export default function SwipeDeck({
   return (
     <>
       {!allSwiped && !resolvedMatchId && (
-        <div className="flex items-center justify-center gap-2 px-4 pb-1">
+        <div className="mx-auto flex w-full max-w-sm items-center gap-2 px-4 pb-1">
-          <div className="h-1 flex-1 max-w-sm overflow-hidden rounded-full bg-zinc-100">
+          <div className="h-1 flex-1 overflow-hidden rounded-full bg-zinc-100">
             <motion.div
               className="h-full rounded-full bg-emerald-400"
               initial={{ width: 0 }}
@@ -161,6 +185,14 @@ export default function SwipeDeck({
           <span className="shrink-0 text-[11px] tabular-nums text-zinc-400">
             {currentIndex}/{restaurants.length}
           </span>
+          <button
+            onClick={handleUndo}
+            disabled={currentIndex === 0}
+            className="flex shrink-0 items-center gap-0.5 rounded-full px-1.5 py-0.5 text-[11px] font-medium text-amber-500 transition-colors active:bg-amber-50 disabled:opacity-0"
+          >
+            <Undo2 size={12} />
+            撤回
+          </button>
         </div>
       )}