fix(channel): allow attachment-only messages via WebSocket (#331)

The WebSocket handler rejected messages with empty text even when
attachments were present, while the HTTP POST endpoint correctly
used Message.IsEmpty(). Move the empty-check after attachment
parsing so only truly empty messages are rejected.

internal/handlers/local_channel.go

@@ -392,10 +392,6 @@ func (h *LocalChannelHandler) HandleWebSocket(c echo.Context) error {
 
 		case "message":
 			text := strings.TrimSpace(msg.Text)
-			if text == "" {
-				writer.SendJSON(map[string]string{"type": "error", "message": "message text is required"})
-				continue
-			}
 
 			chatAttachments := make([]conversation.ChatAttachment, 0, len(msg.Attachments))
 			for _, rawAtt := range msg.Attachments {
@@ -405,6 +401,11 @@ func (h *LocalChannelHandler) HandleWebSocket(c echo.Context) error {
 				}
 			}
 
+			if text == "" && len(chatAttachments) == 0 {
+				writer.SendJSON(map[string]string{"type": "error", "message": "message text or attachments required"})
+				continue
+			}
+
 			// Drain any previous abort signal.
 			select {
 			case <-abortCh: