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/services/garmin.py

@@ -1111,6 +1111,20 @@ def sync_data(user_id, creds, days=None, client=None, trigger="manual"):
 
     try:
         client = client or _connect(creds, user_id)
+    except RateLimited as e:
+        # A genuine 429 at connect time: _connect already wrote the fresh
+        # cooldown, so surface this as rate_limited (the UI draws a distinct
+        # stand-down state for it) rather than a generic error.
+        message = str(e)
+        _set_sync_status(user_id, "rate_limited", now, records_synced=0,
+                         last_error=message)
+        return finish({
+            "status": "rate_limited",
+            "recordsSynced": 0,
+            "message": message,
+            "mfaRequired": False,
+            "lastSyncTime": now,
+        })
     except Exception as e:
         message = describe(e)
         _set_sync_status(user_id, "error", now, records_synced=0, last_error=message)