Refactor view handling and keybinding API

Move the `View` enum to a dedicated module and implement `Widget` for
`&App` there.
Rename `default_view_keybindings` to `default_keybindings` and adjust
its
signature. Redesign `KeybindingsWidget` to accept an explicit list of
bindings instead of a `View`. Update imports, function signatures, and
rendering
logic across the codebase to reflect these changes.

src/app/views/mod.rs

@@ -0,0 +1,7 @@
+pub mod default;
+pub mod main_menu;
+pub mod settings;
+
+pub use default::default_view;
+pub use main_menu::main_menu_view;
+pub use settings::settings_view;