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

Cargo.lock

@@ -16878,18 +16878,22 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "client",
+ "component",
  "db",
+ "documented",
  "editor",
  "fuzzy",
  "gpui",
  "install_cli",
  "language",
+ "linkme",
  "picker",
  "project",
  "schemars",
  "serde",
  "settings",
  "telemetry",
+ "theme",
  "ui",
  "util",
  "vim_mode_setting",