Add systemd service ordering dependencies to prevent EasyEffects from
starting before the Wayland compositor is ready.
**Problem:** EasyEffects would crash during login with "Failed to create
wl_display (No such file or directory)" if started before the Wayland
display server was fully initialized.
**Solution:** Declaring the "After" property ensures EasyEffects waits
for the graphical session to be fully ready. The "PartOf" properly stops
EasyEffects when logging out. Got the tip from
https://github.com/wwmm/easyeffects/issues/1310.
Tested on NixOS 25.11 (Xantusia, 20251223.76701a1) and Home Manager 25.11
(0999ed8) with GNOME 49 on Wayland.
4067ca1ffb