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/view.rs

@@ -0,0 +1,28 @@
+use crate::app::{
+    App,
+    views::{default_view, main_menu_view, settings_view},
+};
+use clap::ValueEnum;
+use ratatui::{buffer::Buffer, layout::Rect, widgets::Widget};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]
+pub enum View {
+    MainMenu,
+    Skirmish,
+    PerkDecks,
+    SkillsConfig,
+    Settings,
+}
+
+impl Widget for &App {
+    fn render(self, area: Rect, buf: &mut Buffer)
+    where
+        Self: Sized,
+    {
+        match self.view {
+            View::MainMenu => main_menu_view(self, area, buf),
+            View::Settings => settings_view(self, area, buf),
+            _ => default_view(area, buf),
+        }
+    }
+}