Display stored size instead of backend usage in UI

[misterflop]

Current behavior In the UI, the size shown for a Kloset is based on the effective storage usage on the backend. This works on filesystems where we can inspect disk usage, but it is not available on all backends (e.g. S3, FTP, SFTP).

Problem

• Backend usage is not portable across storage types.
• This metric may be missing or misleading depending on the backend.
• Users cannot compute Plakar’s efficiency ratio (deduplication + compression) consistently if the reported size depends on backend capabilities.

Requested change Change the UI so that it displays the stored size instead of the backend’s physical usage:

• Logical size (unchanged): Still represents the full expanded size of all snapshots after decompression and deduplication.
• Stored size (replaces current “storage size”): Represents the aggregate amount of data written into the Kloset from the last commit.
• Independent of backend capabilities, available on all storage types. Can be used consistently to compute efficiency ratios.

(Optional) Backend size:

May still be shown as a secondary metric when available, but not as the primary value.

Example UI change

Instead of: Kloset A: 12.3 GB

Show: Kloset A:

• Logical size: 25.4 GB
• Stored size: 12.3 GB
• Backend size: 11.8 GB (optional, only when supported)

Benefits

• Works across all backends (filesystem, S3, FTP, SFTP, …).
• Enables consistent calculation of Plakar’s efficiency ratio.
• Avoids confusion caused by backend-specific reporting.