fix(swayosd): ensure swayosd starts reliably

- Add explicit 'systemctl --user start swayosd' to Hyprland exec-once
- Harden systemd service definition with proper dependencies and targets

apps/hyprland/hypr/exec.nix

@@ -15,6 +15,7 @@
       # essentials
       "hyprpm reload -n"
       "systemctl --user start hyprpolkitagent"
+      "systemctl --user start swayosd"
       "hyprsunset -t 4500" # TODO: must be optional
     ];
   };