[Penpot] Merge Assets and Tokens and redesign them

### Is your feature request related to a problem? Please describe. There is no real reason to them to be separate thing. Assets doesn't have the same functionality as Tokens, and Tokens aren't included in Libraries As Sets aren't the worst idea, as it makes sense that you would want to change theme for everything, it works bad, it's a lot of duplicate works, there is no subthemes. I'm not sure if it's good thing to be able to set every value in sets (like "Color", "Border Radius") We probably should allow easy way to set variants in there. Selecting colors from "Color tokens" should be redesigned to as it's just bad: <img width="741" height="630" alt="Image" src="https://github.com/user-attachments/assets/9cb137fb-7fa9-4d62-9048-26ab2302ced0" /> ### Describe the solution you'd like. Something like in figma, where it is one thing. <img width="752" height="1228" alt="Image" src="https://github.com/user-attachments/assets/aa3c9372-7aa4-43ed-940e-82de1199a0f6" /> There is s