Fix WebSocket 403 in dev mode by allowing localhost origins

The CheckOrigin function only accepted the production domain, rejecting
localhost connections in dev. Also removed redundant error response after
a failed upgrade since the upgrader already writes its own HTTP response.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/handlers/handle_websocket.go

@@ -8,7 +8,7 @@ import (
 func (store *Store) ConnectWebSocket(ctx *gin.Context) {
 	conn, err := services.Upgrader.Upgrade(ctx.Writer, ctx.Request, nil)
 	if err != nil {
-		ctx.JSON(500, gin.H{"error": err.Error()})
+		// Upgrader already wrote the HTTP error response, so just return
 		return
 	}