welcome: Theme preview tile (#29689)

![CleanShot 2025-04-30 at 13 26
44@2x](https://github.com/user-attachments/assets/f68fefe2-84a1-48b7-b9a2-47c2547cd06b)


- Adds the ThemePreviewTile component, used for upcoming onboarding UI
- Adds the CornerSolver utility for resolving correct nested corner
radii

Release Notes:

- N/A

crates/ui/src/utils.rs

@@ -4,11 +4,13 @@ use gpui::App;
 use theme::ActiveTheme;
 
 mod color_contrast;
+mod corner_solver;
 mod format_distance;
 mod search_input;
 mod with_rem_size;
 
 pub use color_contrast::*;
+pub use corner_solver::{CornerSolver, inner_corner_radius};
 pub use format_distance::*;
 pub use search_input::*;
 pub use with_rem_size::*;