Front-end and design is one thing. Get better at them, and build your career on that. On the other hand, companies will usually try to box you in to one of those disciplines. Don't underestimate how insane they will get as they try to do it. If they succeed, then either way you should always find time to pursue other interests, and to develop other skills, and to grow in your main field, or else one day you'll be left ass out with nowhere to go. What you know now can become old news very quickly.

For your future to be bright, you should retain this "duality." And branch off on your own one day (before the ship sails).

Specifically about the front-end vs. design question, what I've seen is that most design+front-end folks reach a maturation point with the front-end technologies (and it becomes mostly about keeping up), then spend the rest of their time further developing their design skills.

I'll say it. I think that the best or most successful future tech entrepreneurs will be the Front-end Engineers and the Marketers. They are sitting there on the edge everyday with the users, and (overtime) they intuitively learn exactly what matters.

Most of the others fail.