Extract built-in theme data from theme.rs into theme_builtins.rs

Moves ~265 lines of TOML theme constant data to a dedicated file,
reducing theme.rs from 553 to ~290 lines of logic.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/ui/mod.rs

@@ -5,6 +5,7 @@ pub mod header;
 pub mod layout;
 pub mod pages;
 pub mod theme;
+mod theme_builtins;
 pub mod widgets;
 
 pub use layout::draw;