Fix Niri focus detection with tmux

[pernilsalat]

Fix Niri focus detection with tmux

Fixes #170

This changes focus detection so that Niri and tmux focus state are considered together.

Previously, when the Niri window containing tmux remained focused, the Niri-specific branch returned before tmux focus detection could determine whether the original tmux window was still active.

The Niri check now returns early only when the Niri window is no longer focused. Otherwise, focus detection continues to the tmux check.

The tmux check also includes pane_active, allowing done to distinguish between panes within the same tmux window.

Tested manually with:

• same Niri window and same tmux pane: no notification
• same Niri window and different tmux pane: notification
• same Niri window and different tmux window: notification
• different Niri window: notification
• outside tmux, same Niri window: no notification
• outside tmux, different Niri window: notification