Run breadd as a systemd user service by default

Best practice for the long-running bread daemon: ship an enabled user unit in
skel (~/.config/systemd/user/breadd.service + default.target.wants symlink)
instead of a bare Hyprland exec-once. Gives crash-restart, journald logging
(journalctl --user -u breadd), and proper lifecycle.

- ExecStart uses %h so it works for any account created from skel (not a
  hardcoded home).
- RuntimeDirectoryPreserve=yes so restarting breadd doesn't wipe the shared
  theme.css that bread-theme writes into /run/user/<uid>/bread.
- hyprland.lua: replace the `breadd` exec-once with a Wayland-env import
  (dbus-update-activation-environment) + `systemctl --user restart breadd`, so
  the service — which autostarts at login before Hyprland exists — picks up
  HYPRLAND_INSTANCE_SIGNATURE and can drive the compositor.

iso/airootfs/etc/skel/.config/hypr/hyprland.lua

@@ -203,7 +203,12 @@ hl.on("hyprland.start", function()
         "awww-daemon",
         -- set the default wallpaper once the daemon is up (retry until ready)
         [[bash -c 'until awww img /usr/share/backgrounds/bos/bread-background.png 2>/dev/null; do sleep 0.3; done']],
-        "breadd",
+        -- breadd runs as a systemd user service (~/.config/systemd/user/breadd.service,
+        -- enabled in skel). It autostarts at login but before Hyprland exists, so
+        -- push the compositor's Wayland env into the user manager and restart breadd
+        -- to pick it up — that's how it gets HYPRLAND_INSTANCE_SIGNATURE to talk to Hyprland.
+        "dbus-update-activation-environment --systemd WAYLAND_DISPLAY XDG_CURRENT_DESKTOP HYPRLAND_INSTANCE_SIGNATURE",
+        "systemctl --user restart breadd",
         "breadbar",
         "breadbox-sync",
         "hypridle",