Introduce central theme module; rounded borders and consistent styling

All colors/styles now live in src/theme.rs (palette + pane/dialog
block builders): sidebar, file view, bottom bar, context menu and all
dialogs share one look that can be changed in one place. Rounded
borders throughout.

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

src/main.rs

@@ -1,6 +1,7 @@
 mod app;
 mod config;
 mod file_ops;
+mod theme;
 mod ui;
 mod events;