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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

iso/airootfs/etc/skel/.config/systemd/user/breadd.service

@@ -0,0 +1,21 @@
+[Unit]
+Description=Bread Runtime Daemon
+
+[Service]
+Type=simple
+# %h = the user's home — works for any account created from this skel.
+ExecStart=%h/.local/bin/breadd
+Restart=on-failure
+RestartSec=2
+UMask=0077
+RuntimeDirectory=bread
+RuntimeDirectoryMode=0700
+# Keep /run/user/<uid>/bread across restarts so the shared theme.css that
+# bread-theme writes there (and the daemon socket) survive a `restart breadd`.
+RuntimeDirectoryPreserve=yes
+KillSignal=SIGTERM
+TimeoutStopSec=5
+Environment=RUST_LOG=info
+
+[Install]
+WantedBy=default.target