fix: KuCoin timestamp sync, improve no-coins handling, update README icons

- Add server time synchronization for KuCoin API to fix timestamp error (400002)
- Return empty list instead of error when no available coins (ai500.go)
- Save cycle record even when no candidate coins (show in frontend without red error)
- Update Claude icon to Anthropic dark brand color (#141413)
- Add exchange and AI model icons to README.md and README.ja.md

provider/nofxos/ai500.go

@@ -105,7 +105,8 @@ func (c *Client) GetTopRatedCoins(limit int) ([]string, error) {
 	}
 
 	if len(availableCoins) == 0 {
-		return nil, fmt.Errorf("no available coins")
+		// Empty list is normal - just return empty slice, not an error
+		return []string{}, nil
 	}
 
 	// Sort by Score descending (bubble sort)
@@ -147,10 +148,7 @@ func (c *Client) GetAvailableCoins() ([]string, error) {
 		}
 	}
 
-	if len(symbols) == 0 {
-		return nil, fmt.Errorf("no available coins")
-	}
-
+	// Empty list is normal - just return empty slice, not an error
 	return symbols, nil
 }