fix: 连接阶段真实 429 以 rate_limited 状态呈现，而非泛化 error

sync_data 的 _connect 抛 RateLimited（真实 429，冷却已写入）原本落进
except Exception 返回 status=error，message 带 'RateLimited:' 前缀——前端
SettingsPage 只对 status=rate_limited 渲染'被限流'提示，导致放开本地拦截
后真实 429 的提示错位。

单独捕获 RateLimited：status=rate_limited + 干净 message + history 记录
rate_limited。补测试 test_a_real_429_at_connect_surfaces_as_rate_limited
（597 passed）

backend/tests/test_garmin_sync.py

@@ -745,6 +745,26 @@ class TestRateLimiting:
         garmin_svc._connect({}, user_id=user["id"])
         assert calls == [], "an unexpired token needs no refresh"
 
+    def test_a_real_429_at_connect_surfaces_as_rate_limited(self, db, user, monkeypatch):
+        """A genuine 429 while connecting is a stand-down, not a generic
+        failure: sync_data must report status rate_limited so the UI draws the
+        distinct '被限流' state instead of a plain error."""
+        def throttled(_creds, _uid=None):
+            garmin_svc._note_rate_limit(user["id"])  # what _connect does on 429
+            raise garmin_svc.RateLimited(
+                "Garmin 暂时限制了请求频率。这通常是短时间内连接过于频繁，"
+                "等待约半小时后会自动恢复，令牌本身没有失效。"
+            )
+
+        monkeypatch.setattr(garmin_svc, "_connect", throttled)
+        out = garmin_svc.sync_data(user["id"], CREDS, days=1)
+
+        assert out["status"] == "rate_limited"
+        assert "令牌本身没有失效" in out["message"]
+        assert garmin_svc.get_sync_status(user["id"])["status"] == "rate_limited"
+        assert garmin_svc.get_sync_history(user["id"])[0]["status"] == "rate_limited"
+        garmin_svc._rate_limited_until.clear()
+
     def test_a_valid_token_is_not_refreshed(self, db, user, monkeypatch):
         """Refreshing on every connect spends quota for nothing — and that is
         what walked the account into a 429 in the first place."""