Adopt clap::ValueEnum for Group and tidy menu view

- Derive `ValueEnum` for `Group` and remove its custom `iter` method.
- Update keybindings widget to use `Group::value_variants()` and adjust
  containment and filtering logic.
- Replace view option generation with a `format_view_string` helper that
  inserts a space before the second capital letter, eliminating the
  previous filter for “MAIN MENU”.
- Adjust imports to include `clap::ValueEnum` where needed.

src/app/keybindings/keybindings.rs

@@ -1,3 +1,4 @@
+use clap::ValueEnum;
 use ratatui::crossterm::event::{KeyCode, KeyEvent, KeyEventKind, KeyModifiers};
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
@@ -11,21 +12,13 @@ pub enum Action {
     Esc,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, ValueEnum)]
 pub enum Group {
     Movement,
     Select,
     Quit,
 }
 
-impl Group {
-    pub fn iter() -> impl Iterator<Item = Group> {
-        [Group::Movement, Group::Select, Group::Quit]
-            .iter()
-            .copied()
-    }
-}
-
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 pub struct KeyBinding {
     pub action: Action,