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::*;

crates/ui/src/utils/corner_solver.rs

@@ -0,0 +1,61 @@
+use gpui::Pixels;
+
+/// Calculates the child’s content-corner radius for a single nested level.
+///
+/// child_content_radius = max(0, parent_radius - parent_border - parent_padding + self_border)
+///
+/// - parent_radius: outer corner radius of the parent element
+/// - parent_border: border width of the parent element
+/// - parent_padding: padding of the parent element
+/// - self_border: border width of this child element (for content inset)
+pub fn inner_corner_radius(
+    parent_radius: Pixels,
+    parent_border: Pixels,
+    parent_padding: Pixels,
+    self_border: Pixels,
+) -> Pixels {
+    (parent_radius - parent_border - parent_padding + self_border).max(Pixels::ZERO)
+}
+
+/// Solver for arbitrarily deep nested corner radii.
+///
+/// Each nested level’s outer border-box radius is:
+///   R₀ = max(0, root_radius - root_border - root_padding)
+///   Rᵢ = max(0, Rᵢ₋₁ - childᵢ₋₁_border - childᵢ₋₁_padding) for i > 0
+pub struct CornerSolver {
+    root_radius: Pixels,
+    root_border: Pixels,
+    root_padding: Pixels,
+    children: Vec<(Pixels, Pixels)>, // (border, padding)
+}
+
+impl CornerSolver {
+    pub fn new(root_radius: Pixels, root_border: Pixels, root_padding: Pixels) -> Self {
+        Self {
+            root_radius,
+            root_border,
+            root_padding,
+            children: Vec::new(),
+        }
+    }
+
+    pub fn add_child(mut self, border: Pixels, padding: Pixels) -> Self {
+        self.children.push((border, padding));
+        self
+    }
+
+    pub fn corner_radius(&self, level: usize) -> Pixels {
+        if level == 0 {
+            return (self.root_radius - self.root_border - self.root_padding).max(Pixels::ZERO);
+        }
+        if level >= self.children.len() {
+            return Pixels::ZERO;
+        }
+        let mut r = (self.root_radius - self.root_border - self.root_padding).max(Pixels::ZERO);
+        for i in 0..level {
+            let (b, p) = self.children[i];
+            r = (r - b - p).max(Pixels::ZERO);
+        }
+        r
+    }
+}