Add new user agent

[suchmememanyskill]

Please tick as appropriate:

• [ ] I have tested this code on a steam deck or on a PC
• [X] My changes generate no new errors/warnings
• [X] This is a bugfix/hotfix
• [ ] This is a new feature

If you're wanting to update a translation or add a new one, please use the weblate page: https://weblate.werwolv.net/projects/decky/

Description

This fixes issue: The new beta steam client changes the user agent. This causes decky to close browser tabs it shouldn't. This PR fixes that