Add Settings view, widget, and keybindings

Introduce Settings view handling in key events and widgets.
Add `Enter` action with its keybinding.
Refactor keybinding lookup to use `Option<&KeyBinding>` for missing
entries.
Update view option generation to safely filter possible values.
Export `settings_keybindings` and `settings_widget` from their modules.

src/app/keybindings/settings.rs

@@ -0,0 +1,16 @@
+use crate::app::{
+    App, View,
+    keybindings::{Action, event_to_action},
+};
+use ratatui::crossterm::event::KeyEvent;
+
+pub fn settings_keybindings(app: &mut App, key_event: &KeyEvent) {
+    if let Some(action) = event_to_action(&key_event) {
+        match action {
+            Action::Quit => app.exit = true,
+            Action::Quit2 => app.exit = true,
+            Action::Esc => app.view = View::MainMenu,
+            _ => (),
+        }
+    }
+}