KeyboardFocus should be visually clearly indicated for better user experience, which is in Aero but Fluent regresses

### Summary

In recent WPF themes (e.g., Fluent), `KeyboardFocus` may be present but not visually indicated. This is a regression from older themes like Aero and Classic, which provided clear, predictable focus feedback.

### Why this is a design flaw

- Users cannot determine which control will respond to the next key press.
- Aero provided a clear visual indicator via `FocusVisualStyle` and animation.
- Fluent omits this, leading to ambiguity and reduced accessibility.

### Suggested Resolution

- Reinstate visual feedback for `KeyboardFocus` in default templates.
- Use `FocusVisualStyle` or `IsKeyboardFocused` triggers to provide consistent feedback.

### Philosophical Note

A UI that accepts input without revealing its focus state violates the principle of explicit responsibility modeling. Aero got this right. Fluent regressed.