feat: add ZFS monitoring (#2209)

- track pool capacity, health, I/O, scrub status, and vdev errors
- report dataset usage and correct ZFS filesystem metrics
- add pool charts, detail views, refresh controls, and health alerts
- persist pool details and include ZFS usage in disk alerts
- support configurable detail intervals and legacy agent compatibility

---------

Co-authored-by: hank <hank@henrygd.me>

agent/zfs/zfs_freebsd.go

@@ -3,9 +3,17 @@
 package zfs
 
 import (
+	"errors"
+
 	"golang.org/x/sys/unix"
 )
 
 func ARCSize() (uint64, error) {
 	return unix.SysctlUint64("kstat.zfs.misc.arcstats.size")
 }
+
+// FreeBSD does not expose Linux's per-pool procfs kstats. Capacity, health,
+// and detail collection still work through the cached utilities.
+func PoolKernelStats() ([]PoolKernelStat, error) {
+	return nil, errors.ErrUnsupported
+}