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

Cargo.lock

@@ -279,7 +279,7 @@ checksum = "a1d728cc89cf3aee9ff92b05e62b19ee65a02b5702cff7d5a377e32c6ae29d8d"
 
 [[package]]
 name = "cm-dashboard"
-version = "0.1.265"
+version = "0.1.267"
 dependencies = [
  "anyhow",
  "chrono",
@@ -301,7 +301,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-agent"
-version = "0.1.265"
+version = "0.1.267"
 dependencies = [
  "anyhow",
  "async-trait",
@@ -325,7 +325,7 @@ dependencies = [
 
 [[package]]
 name = "cm-dashboard-shared"
-version = "0.1.265"
+version = "0.1.267"
 dependencies = [
  "chrono",
  "serde",