Follow-up Automatic Directional Navigation for input focus

[jbuehler23]

Add automatic layer navigation support (follow-on from automatic directional navigation)

Currently, the algorithm is z-index agnostic - it treats all entities with AutoDirectionalNavigation as a flat set based purely on XY position. So overlapping windows/layers would incorrectly connect to each other.

For proper layer handling, you'd need to:

1. Query entities per-layer manually and call auto_generate_navigation_edges() separately for each layer, OR
2. Use manual edges to explicitly define cross-layer navigation when desired

To avoid making this PR too large, I would add automatic layer support as a future enhancement. You could:

• Filter by TabGroup component, as suggested!
• Add layer_id field to AutoDirectionalNavigation
• Use parent entity hierarchy to detect separate UI trees

Would like to know if you think it's necessary for this PR to address this, or is a follow-up PR better?

Originally posted by @jbuehler23 in #21668 (comment)

Enable input focs by default - based on @alice-i-cecile feedback on PR #21668

From auto_directional_navigation example, we should enable the input focus plugins by default:

App::new()
        // Input focus is not enabled by default, so we need to add the corresponding plugins
        .add_plugins((
            DefaultPlugins,
            InputDispatchPlugin,
            DirectionalNavigationPlugin,
        ))

Rename TabGroup to FocusGroup (per @viridia 's suggestion)

[viridia]

Suggestion: we could rename TabGroup to FocusGroup (no pun intended) or InputFocusGroup to make it clearer that it's not just about tabs.