Css variables enhancements

[TrySound]

• filters and backdrop filters still do not support variables
• rename variables
• search properties in advanced panel
• gradient thumbnail in variables (requires portable gradient ui)
• preview highlighted variables in css fragment autocomplete (background and shadow sections)
• preview computed values with pressed alt in css inputs
• support vars in individual border size/radius fields (it somewhat works now)

Ideas

• Prevent Marketplace insert from overriding CSS var values, only allow it to add new variables.
• Render computed value in spacing
• Add expand option like we have for code
• Var wrapping doesn't look great IMO but cutting off is probably worse
• Might make sense to add a copy option on single var... I would need to build a site using this to see my workflow but I could see me forgetting my vars, referencing the global list, then wanting to copy it. Though maybe autocomplete will solve this (autocomplete indeed does solve this)
• Remove var() from field display and just show --size-6. It adds a lot of clutter and often hides the details
• Alphabetize or maintain order on undo after removing a variable. I accidentally removed one and upon undo, it gets put at the top
  https://github.com/user-attachments/assets/37bd6161-d352-4e5d-8565-0431a855738e