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>

internal/entities/zfs/zfs.go

@@ -0,0 +1,45 @@
+// Package zfs defines the ZFS detail data exchanged between agent and hub.
+package zfs
+
+// ZfsData is the detail payload returned by the agent for the GetZfsData action.
+type ZfsData struct {
+	Pools    []*PoolDetail `json:"pools,omitempty"`
+	Complete bool          `json:"complete,omitempty"`
+}
+
+// PoolDetail holds the verbose state of a single pool: capacity, health,
+// scrub, vdev, and dataset information.
+type PoolDetail struct {
+	Name     string     `json:"name"`
+	Health   string     `json:"health,omitempty"`
+	Size     uint64     `json:"size,omitempty"`  // bytes
+	Alloc    uint64     `json:"alloc,omitempty"` // bytes
+	Free     uint64     `json:"free,omitempty"`  // bytes
+	Scrub    *Scrub     `json:"scrub,omitempty"`
+	Vdevs    []*Vdev    `json:"vdevs,omitempty"`
+	Datasets []*Dataset `json:"datasets,omitempty"`
+}
+
+// Scrub holds the scrub (or resilver) status of a pool.
+type Scrub struct {
+	State    string `json:"state,omitempty"` // NONE, SCANNING, FINISHED, CANCELED
+	Progress string `json:"progress,omitempty"`
+	Errors   uint64 `json:"errors,omitempty"`
+}
+
+// Vdev is a single vdev (mirror, raidz, or leaf disk) with error counters.
+type Vdev struct {
+	Name         string `json:"name"`
+	State        string `json:"state,omitempty"`
+	ReadErrs     uint64 `json:"readErrs,omitempty"`
+	WriteErrs    uint64 `json:"writeErrs,omitempty"`
+	ChecksumErrs uint64 `json:"checksumErrs,omitempty"`
+}
+
+// Dataset is a single ZFS dataset with usage information.
+type Dataset struct {
+	Name       string `json:"name"`
+	Used       uint64 `json:"used,omitempty"`
+	Avail      uint64 `json:"avail,omitempty"`
+	Mountpoint string `json:"mount,omitempty"`
+}