Implement focus management for windows

[pbalduino]

Summary

Implement window focus management, including click-to-focus, keyboard event routing, and focus visual indicators.

Goals

• Click-to-focus behavior
• Keyboard event routing to focused window
• Focus visual feedback
• Focus-follows-mouse (optional)
• Tab cycling between windows

Implementation

Focus Management

```c
struct window *focused_window = NULL;

void wm_focus_window(struct window *w) {
if (focused_window) {
focused_window->focused = 0;
wm_redraw_decoration(focused_window); // Inactive color
}

focused_window = w;
w->focused = 1;
wm_raise_window(w);
wm_redraw_decoration(w);  // Active color

send_focus_event(w);

}

void wm_handle_mouse_click(int x, int y) {
struct window *w = wm_find_window_at(x, y);
if (w && w != focused_window) {
wm_focus_window(w);
}
}
```

Timeline

Total: 1-2 weeks

Definition of Done

• Click-to-focus works
• Keyboard events go to focused window
• Visual focus indicator
• Tab cycling works
• Focus events sent to applications

Dependencies

• Implement composition engine with damage tracking #405: Window manager core
• Implement GUI input event system #399: Input events

See docs/road/road_to_gui.md for complete roadmap.