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/app.rs

@@ -27,7 +27,7 @@ pub struct App {
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]
 pub enum GameMode {
     LastManStanding,
-    Frontlines,
+    FrontLines,
 }
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]