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/mod.rs

@@ -1,5 +1,9 @@
 pub mod app;
+pub mod keybindings;
 pub mod widget;
 pub mod widgets;
 
-pub use app::{App, Event, handle_input_events};
+pub use app::{App, Event, View, handle_input_events};
+pub use keybindings::{
+    Action, KEYBINDINGS, KeyBinding, binding_for, binding_for_view, event_to_action,
+};