Refactor settings UI and add typed option handling

- Rename `GameMode::Frontlines` to `FrontLines`.
- Introduce `SettingsValue` enum and `SettingsOption` vector for
  typed settings.
- Implement `Display` for `SettingsValue`, `GameMode`, and
  `PerkDecks`.
- Enhance settings keybindings: navigation with Up/Down, value
  parsing on Enter, and error messages.
- Update settings view to render options list, selection marker,
  and popup with error feedback.
- Restyle main menu selector and keybindings widget title.
- Add required imports and minor layout adjustments.

src/app/widgets/keybindings.rs

@@ -72,7 +72,11 @@ impl Widget for KeybindingsWidget {
 
         let block: Block<'_> = Block::default()
             .borders(Borders::ALL)
-            .title("[ Keybindings ]");
+            .title(Line::from_iter([
+                "[ ".gray(),
+                "Keybindings".magenta(),
+                " ]".gray(),
+            ]));
 
         let inner: Rect = block.inner(area);