fix: report accurate storage usage and disk ownership

[alvins82]

Summary

• Traverse nested lsblk trees so filesystems under partitions and LVM logical volumes appear in Storage Manager.
• Report actual mounted filesystem used and available space for dashboard and USB storage statistics.
• Preserve the physical disk model and path for each storage entry.
• Add coverage for nested mounts and logical volumes.

The previous behavior treated a physical disk's allocated partition/LVM size as used space and missed the nested root filesystem. This change reports filesystem usage from the mounted filesystems themselves.

Validation:

• GOOS=linux GOARCH=amd64 go build ./...
• GOOS=linux GOARCH=amd64 go test -c ./service
• git diff --check

This draft PR intentionally targets alvins82/CasaOS-LocalStorage only; it does not target IceWhaleTech upstream.