iso: bake bakery apps into /usr/local

BOS opts in to bakery's system prefix so desktop apps live on @
and ride snapper/grub-btrfs snapshots. The builder home stays
~/.local; build-local.sh copies bins, share/data, and user units
onto the image. Per-user installed.json and the index cache stay
in skel. Recovery is still grub-btrfs, not snapper rollback.

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

@@ -3,8 +3,8 @@ 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
+# System-prefix bakery install — same path for every account.
+ExecStart=/usr/local/bin/breadd
 Restart=on-failure
 RestartSec=2
 UMask=0077