Implement multi-disk backup support

- Update BackupData structure to support multiple backup disks
- Scan /var/lib/backup/status/ directory for all status files
- Calculate status icons for backup and disk usage
- Aggregate repository status from all disks
- Update dashboard to display all backup disks with per-disk status
- Display repository list with count and aggregated status

CLAUDE.md

@@ -327,9 +327,16 @@ Storage:
   ├─ ● Data_2: GGA04461 T: 28°C
   └─ ● Parity: WDZS8RY0 T: 29°C
 Backup:
+● Repo: 4
+  ├─ getea
+  ├─ vaultwarden
+  ├─ mysql
+  └─ immich
+● W800639Y W: 2%
+  ├─ ● Backup: 2025-11-29T04:00:01.324623
+  └─ ● Usage: 8% 70GB/916GB
 ● WD-WCC7K1234567 T: 32°C W: 12%
-  ├─ Last: 2h ago (12.3GB)
-  ├─ Next: in 22h
+  ├─ ● Backup: 2025-11-29T04:00:01.324623
   └─ ● Usage: 45% 678GB/1.5TB
 ```