What CS / IT skills, apps, tech, etc would be most beneficial for the leftist movement?

Learn to organize (workplace, housing, etc. etc.). I'm not kidding. Way more important than any code-related skills you're learning in school from the POV of benefiting the proletariat, and something woefully lacking in most coders despite our coming proletarianization being obvious. You can join the IWW and they'll teach you for free.

Actual technical skills? Probably infosec stuff. Do CTFs, etc.

Very little of the rest of it is useful for proles in a world ruled by the bourgeoisie. And once that's not the case, just pursue what interests you in the same way you would any career as a software engineer because it'll all be useful. Could also look at emigrating to an AES state where that's already true.

Sysadmin shit, to host independent communities and servers. Not cloud BS, hosting on dedicated servers instead - from at least slightly more censorship-resistant hosts - or even home servers (greater legal risk), and load-balancing properly. High level (as in: removed from the math, not "advanced" - just get a handle on what's secure) encryption stuff, to implement secure communication that does not rely on an easy common target (custom apps potentially, including mobile apps for the zoomers - even if they're mostly just a chat frondend to GPG; I recommend user-enabled NFC key sharing if you go this route - tap a few buttons, put your smartphone against a comrade's phone, boom, trusted key exchange - do not trust smartphones for anything too critical though, and the same applies to windows computers or linux ones without proper full-disk encryption). Webshit stuff, to publish content and try and inform people (though mind you on that one wordpress would almost universally do the job). Cybersecurity stuff, finally, for data extraction and hacktivism and the like - auditing code (or fuzztesting proprietary binaries) for flaws; not publishing them straight out and exploiting them to disrupt shit or at the very least diffuse a message.

Basically what @captcha said

Put your time into getting a decent job where you do as little work as possible and use your bazinga bucks / limitless vacation policy for organizing

No tech project is going to build socialism. Many Occupy Wall street era people tried it and it went no where. That said, in the course of building a socialist movement IRL, you'll find having tech skills are occaisonally useful, either to have an online presence like a website or to automate tedious computer work.

• deploy a static site with github or gitlab pages
• DNS management.
• manage a linux cloud VM. ssh and bash etc.
• deploy services to a VM with docker-compose. Eg self host wordpress
• Processing datasets like membership lists. VBA, python, R etc.
• web scraping whatever data you'll need.

I cannot emphasize this enough, the less code the better. Keep project small and tight. Try to leverage existing functionality as much as possible.

Cryptography and networking to securely communicate during organization and monitor suspicious traffic

General OSINT skills to vet people and provide proof of any claims

Not a CS skill, but common in security work is social engineering, and hell, maybe even a little bit of journalism/blogging. Build up a reputation of some sort and attempt to find sympathetic/disillusioned sources who might expose some stuff that will help protestors/organizers

Web design/UX maybe? These days a lot of people are used to or drawn to “modern” and “sleek” designs. Marxists.org will likely deter new people lol

The first two will probably be the most “useful,” but as others have stated, it’s mostly a lucrative field to make a bunch of money

However, if you want to benefit people (which won’t necessarily further any socialist cause), you can get a job at a public organization such as hospitals and schools. Some libraries are large and popular enough to warrant in house IT staff.

As a software engineer, I asked myself this question a lot when I was in school. Probably, the biggest is enabling secure communication, learning how to hide your footsteps and the footsteps of others. Secure web design, development, hosting is a big one, on top of all of the problem solving techniques you will actually be learning. I find myself benefitting greatly from working on huge projects that would have seemed daunting to me a long time ago, it's a really useful skill to take something very big and break it down into pieces until none is left.

Outside of that, the best thing you could do is organize. Join an organization if you haven't already. Meet people in your community, support striking workers, etc.