Add Context Menu component for right-click triggered menus

[iamnbutler]

Summary

• Adds a new Context Menu component that displays a positioned menu of actions triggered by right-click on a target element
• Implements menu items with labels, icons, keyboard shortcuts, and separators
• Supports disabled and destructive item styles
• Includes keyboard navigation (arrow keys, Enter, Escape)
• Closes menu on click outside or Escape key

Features

• Right-click trigger: Menu appears at cursor position when right-clicking the target element
• Menu items: Support for labels, icons (via closure), keyboard shortcut hints
• Separators: Visual dividers between menu sections
• States: Disabled items (non-interactive) and destructive items (styled in red)
• Keyboard navigation: Arrow keys to move focus, Enter to activate, Escape to close
• Click outside: Closes menu when clicking outside

API Example

let context_menu_state = cx.new(|_cx| {
    ContextMenuState::new(
        context_menu("file-actions")
            .trigger(|_window, cx| {
                div().p_4().child("Right-click me").into_any_element()
            })
            .menu(|_window, _cx| {
                vec![
                    menu_item("copy", "Copy").kbd("Cmd+C").icon(|| Icons::copy()),
                    menu_item("paste", "Paste").kbd("Cmd+V"),
                    menu_separator(),
                    menu_item("delete", "Delete").destructive(),
                ]
            })
            .on_action(|id, _window, _cx| {
                println!("Action: {}", id);
            }),
    )
});

Test plan

• Component compiles without errors
• Showcase example builds and includes context menu demo
• All existing tests pass

Closes #79

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