fix: handle empty AI500 coin list gracefully instead of error

provider/nofxos/ai500.go

@@ -73,8 +73,10 @@ func (c *Client) fetchAI500() ([]CoinData, error) {
 		return nil, fmt.Errorf("API returned failure status")
 	}
 
+	// 空列表是正常情况，不是错误
 	if len(response.Data.Coins) == 0 {
-		return nil, fmt.Errorf("coin list is empty")
+		log.Printf("ℹ️  AI500 returned empty coin list (no coins meet criteria currently)")
+		return []CoinData{}, nil
 	}
 
 	// Set IsAvailable flag