fix(focus): fall back to tmux pane state

[velnae]

Summary

• add a Linux focus helper so tmux pane state can be used as a best-effort fallback when no window id is available
• preserve the existing window-id path when compositor focus detection works, and document the fallback in the README
• add focused tests for the new fallback behavior

Why

On my setup, suppressWhenFocused stayed ineffective even after enabling it because Linux window focus detection returned null before tmux state was consulted.

Tested setup where this works

• OS: Linux
• Session type: Wayland
• Desktop: GNOME
• Terminal: Ghostty
• Multiplexer: tmux
• OpenCode notifier version: 0.2.2

With this patch, notifications are suppressed while the current tmux pane is active, which matches the practical expectation for my setup.

Notes

• This is intentionally a best-effort fallback, not a claim of full GNOME Wayland frontmost-window support.
• I could not run bun test locally in this environment because bun is not installed here.