In short, yes you can run a Lightyear Stocks and Shares ISA on your PC!
For more info, we reached out to Wander Rutgers, the UK CEO of Lightyear who said:
Lightyear is available through the mobile applications, and through the web platform on Lightyear.com. The web supports all account types. You can log into your account on Lightyear.com by clicking the ‘Log in’ button on the top right of the screen and entering your details.
You can read more about the different ways of accessing Lightyear here: https://lightyear.com/en-gb/help/getting-verified/do-i-have-to-download-the-app-or-do-you-have-a-website