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>

internal/localapi/capacity_other.go

@@ -0,0 +1,6 @@
+//go:build !linux
+
+package localapi
+
+// statfsCapacity is Linux-only (syscall.Statfs); on other platforms (dev builds) it reports nothing.
+func statfsCapacity(string) (total, used int64, ok bool) { return 0, 0, false }