v0.59.0: report backing device + capacity for a registry-sourced /disks row

Agent-view showed "—" device + no size for a raw (no-PVE-storage) drive because
the registry union row never set backing_device/total_bytes/used_bytes (Observe
drives get those from pvesm status). Resolve BackingDevice via ByUUIDDevicePath +
read capacity via statfsCapacity (build-tagged syscall.Statfs; no-op off-Linux).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,3 +1,15 @@
+## v0.59.0 — report backing device + capacity for a registry-sourced drive in /disks (2026-07-01)
+
+Completes the `/disks` representation for a registry-sourced (raw, no-PVE-storage) drive: the agent-view
+showed "—" for the device and no size bar, because the union row never populated `backing_device` or
+`total_bytes`/`used_bytes` (Observe drives get those from `pvesm status`, which a raw drive has none of).
+
+- **`internal/localapi/disks.go` handleDisks registry union:** resolve `BackingDevice` from the fs-UUID
+  (`storage.ByUUIDDevicePath`) and read capacity via `statfsCapacity` (new build-tagged
+  `capacity_linux.go` = `syscall.Statfs` on the mount; `capacity_other.go` = no-op for dev builds).
+- `go build/vet/test ./...` clean (Linux + Windows dev). Live: the registry drive now shows its device +
+  size in the agent-view, matching the Observe-sourced drives.
+
 ## v0.58.0 — report GuestPath/BoundUnderParent for a registry-sourced drive in /disks (2026-07-01)
 
 Last piece of first-class raw-drive support: the `/disks` union row for a registry-sourced drive (Impl-2a