refactor: 提取 useGeolocation hook 和 joinRoom 工具函数

- useGeolocation: 将 PanicPage 中 ~50 行 GPS 定位逻辑（requestGps + reverseGeocode + 状态管理）提取为独立 hook
- joinRoom: 统一 3 处重复的 POST /api/room/:id/join 调用（room、invite、panic 页面）

src/app/invite/[id]/page.tsx

@@ -12,6 +12,7 @@ import {
   Coffee,
 } from "lucide-react";
 import { getUserId } from "@/lib/userId";
+import { joinRoom } from "@/lib/room";
 import { Skeleton, SkeletonCircle } from "@/components/Skeleton";
 import Button from "@/components/Button";
 import { getSceneConfig } from "@/lib/sceneConfig";
@@ -48,12 +49,7 @@ export default function InvitePage() {
   const handleJoin = async () => {
     setJoining(true);
     try {
-      const userId = getUserId();
-      await fetch(`/api/room/${roomId}/join`, {
-        method: "POST",
-        headers: { "Content-Type": "application/json" },
-        body: JSON.stringify({ userId }),
-      });
+      await joinRoom(roomId, getUserId());
       router.push(`/room/${roomId}`);
     } catch {
       setJoining(false);