feat: 优化定位体验——反向地理编码显示地名，定位失败明确提示

- 新增 /api/location/regeo 接口，通过高德逆地理编码将 GPS 坐标转为地名
- 页面加载时自动定位，成功后显示"当前位置：浦东新区 xxx"
- 定位失败/权限被拒时显示明确提示+重试按钮，不再静默默认上海
- 无可用位置时阻止创建房间，引导用户手动搜索选择地点

src/app/api/location/regeo/route.ts

@@ -0,0 +1,54 @@
+import { NextResponse } from "next/server";
+
+export async function GET(req: Request) {
+  const { searchParams } = new URL(req.url);
+  const lat = searchParams.get("lat");
+  const lng = searchParams.get("lng");
+
+  if (!lat || !lng) {
+    return NextResponse.json(
+      { error: "lat and lng are required" },
+      { status: 400 },
+    );
+  }
+
+  const apiKey = process.env.AMAP_API_KEY;
+  if (!apiKey) {
+    return NextResponse.json(
+      { error: "AMAP_API_KEY not configured" },
+      { status: 500 },
+    );
+  }
+
+  try {
+    const url = new URL("https://restapi.amap.com/v3/geocode/regeo");
+    url.searchParams.set("key", apiKey);
+    url.searchParams.set("location", `${lng},${lat}`);
+    url.searchParams.set("extensions", "base");
+
+    const res = await fetch(url.toString());
+    const data = await res.json();
+
+    if (data.status !== "1" || !data.regeocode) {
+      return NextResponse.json({ name: null });
+    }
+
+    const comp = data.regeocode.addressComponent;
+    const district = comp?.district || comp?.city || "";
+    const township = comp?.township || "";
+    const neighborhood = comp?.neighborhood?.name || "";
+
+    const name = [district, township, neighborhood]
+      .filter(Boolean)
+      .join(" ")
+      .trim();
+
+    return NextResponse.json({
+      name: name || data.regeocode.formatted_address || null,
+      formatted: data.regeocode.formatted_address || null,
+    });
+  } catch (e) {
+    console.error("Regeo error:", e);
+    return NextResponse.json({ name: null });
+  }
+}