Introduce central theme module; rounded borders and consistent styling

All colors/styles for windows, panel, dropdown and desktop now live in
src/theme.rs — restyle the whole WM from one file. Windows and menus
use rounded borders; hover/focus states share one accent style.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main.rs

@@ -7,6 +7,7 @@ mod log;
 mod pty;
 mod render;
 mod server;
+mod theme;
 
 use app::{App, AppIpcOut, WindowContent};
 use config::Config;