ZFS: dedup toggle per pool and dedup option for new datasets

Pool cards show the dedup ratio and can set dedup=on|off on the pool root
dataset (inherited by children); the new-dataset form gets a dedup option.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packaging/README.md

@@ -14,8 +14,11 @@ periodic SMART/ZFS checks with e-mail alerts.
 
 The ZFS page (http://<host>:8090/zfs) can create datasets in a pool — name (nested
 names like media/video are fine), owner uid:gid for the mountpoint, compression,
-optional quota — and destroy datasets that are empty (no data, no children, no
+dedup, optional quota — and destroy datasets that are empty (no data, no children, no
 snapshots; the check is repeated against the live system before "zfs destroy").
+Each pool card also shows the dedup ratio and can turn dedup on/off on the pool
+root dataset (`zfs set dedup=on|off <pool>`, inherited by child datasets; only new
+writes are affected, and the dedup table needs a lot of RAM).
 
 nasmon runs sandboxed (ProtectSystem=strict → own mount namespace), so these
 commands are executed on the host through transient units: