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>

internal/web/server.go

@@ -73,6 +73,7 @@ func New(cfg *config.Store, st *store.Store, ring *history.Ring, sch *sched.Sche
 	s.mux.HandleFunc("GET /zfs", s.zfs)
 	s.mux.HandleFunc("POST /zfs/create", s.zfsCreate)
 	s.mux.HandleFunc("POST /zfs/destroy", s.zfsDestroy)
+	s.mux.HandleFunc("POST /zfs/dedup", s.zfsDedup)
 	s.mux.HandleFunc("GET /alerts", s.alerts)
 	s.mux.HandleFunc("POST /alerts/{id}/{action}", s.alertAction)
 	s.mux.HandleFunc("GET /shares", s.sharesPage)