What technologies, concepts, and patterns should I be focusing on to
ensure I don't crash and burn once I leave the life of freelancing?
What can I do to prepare myself for developing as part of a team?
This is the better part of your question. You've been working alone for a few years, so how do you learn to work with others? Step back and think about all the people you've interacted with in the work that you've done. If you've been doing "web development," you probably haven't been working entirely alone. You might have worked with other freelancers, your clients, and your service providers. These people may not have worked for you, but they were all part of your 'team' in some sense.
There are certainly technical skills that you'll need to acquire in your new job, like how to use the company's bug tracking system, version control system, time reporting system, and so on. Those things are easy to learn, and new employees coming from other companies will go through a similar transition because the systems they used at their previous jobs were likely different.
The more important skills will be your social skills. If you can play nicely with others, make your managers and colleagues look good and feel good, and fulfill your commitments, you'll be fine. There are about a million books on the topic of how to do well at your job; pick out a few that appeal to you, read them, and think about what they have to say. All I Really Need to Know I Learned in Kindergarten could be a good place to start.