Refactor to View enum and add keybindings

Replace the `default_window` string field with a `View` enum throughout
the
application. Introduce a new keybinding module providing `Action`,
`KeyBinding` structs and helper functions, and integrate a keybindings
widget
into the main menu. Update the CLI to accept a `View` value enum and
adjust
imports and event handling accordingly. Add a release profile in
`Cargo.toml` with LTO, single codegen unit, higher optimisation level
and
binary stripping for smaller, faster builds.

src/app/widgets/mod.rs

@@ -1,3 +1,5 @@
+pub mod keybindings;
 pub mod main_menu;
 
+pub use keybindings::keybindings_widget;
 pub use main_menu::main_menu_widget;