Restructure backup display to show per-repository metrics

Remove disk-based backup display and implement repository-centric view
with per-repo archive counts and sizes. Backup now uses NFS storage
instead of direct disk monitoring.

Changes:
- Remove BackupDiskData, add BackupRepositoryData structure
- Display format: "Repo <timestamp>" with per-repo details
- Show archive count and size (MB/GB) for each repository
- Agent aggregates repo data from backup status TOML files
- Dashboard renders repo list with individual status indicators

shared/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cm-dashboard-shared"
-version = "0.1.266"
+version = "0.1.267"
 edition = "2021"
 
 [dependencies]