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/hooks/useGeolocation.ts

@@ -0,0 +1,70 @@
+"use client";
+
+import { useState, useEffect, useCallback } from "react";
+
+export type GpsStatus = "idle" | "locating" | "success" | "failed" | "denied";
+
+type GpsResult =
+  | { ok: true; lat: number; lng: number }
+  | { ok: false; reason: "unsupported" | "denied" | "timeout" | "unknown" };
+
+function requestGps(): Promise<GpsResult> {
+  return new Promise((resolve) => {
+    if (!navigator.geolocation) {
+      resolve({ ok: false, reason: "unsupported" });
+      return;
+    }
+
+    navigator.geolocation.getCurrentPosition(
+      (pos) =>
+        resolve({ ok: true, lat: pos.coords.latitude, lng: pos.coords.longitude }),
+      (err) => {
+        const reason =
+          err.code === err.PERMISSION_DENIED
+            ? "denied"
+            : err.code === err.TIMEOUT
+              ? "timeout"
+              : "unknown";
+        resolve({ ok: false, reason });
+      },
+      { timeout: 8000, enableHighAccuracy: false },
+    );
+  });
+}
+
+async function reverseGeocode(lat: number, lng: number): Promise<string | null> {
+  try {
+    const res = await fetch(`/api/location/regeo?lat=${lat}&lng=${lng}`);
+    const data = await res.json();
+    return data.name || data.formatted || null;
+  } catch {
+    return null;
+  }
+}
+
+export function useGeolocation() {
+  const [status, setStatus] = useState<GpsStatus>("idle");
+  const [coords, setCoords] = useState<{ lat: number; lng: number } | null>(null);
+  const [locationName, setLocationName] = useState<string | null>(null);
+
+  const locate = useCallback(async () => {
+    setStatus("locating");
+    const result = await requestGps();
+    if (result.ok) {
+      setCoords({ lat: result.lat, lng: result.lng });
+      setStatus("success");
+      const name = await reverseGeocode(result.lat, result.lng);
+      if (name) setLocationName(name);
+    } else {
+      setCoords(null);
+      setLocationName(null);
+      setStatus(result.reason === "denied" ? "denied" : "failed");
+    }
+  }, []);
+
+  useEffect(() => {
+    locate();
+  }, [locate]);
+
+  return { status, coords, locationName, retry: locate };
+}