Implement comprehensive backup monitoring and fix timestamp issues

- Add BackupCollector for reading TOML status files with disk space metrics
- Implement BackupWidget with disk usage display and service status details
- Fix backup script disk space parsing by adding missing capture_output=True
- Update backup widget to show actual disk usage instead of repository size
- Fix timestamp parsing to use backup completion time instead of start time
- Resolve timezone issues by using UTC timestamps in backup script
- Add disk identification metrics (product name, serial number) to backup status
- Enhance UI layout with proper backup monitoring integration

dashboard/src/metrics/mod.rs

@@ -121,10 +121,14 @@ pub mod subscriptions {
     
     /// Backup widget metric subscriptions
     pub const BACKUP_WIDGET_METRICS: &[&str] = &[
-        "backup_status",
+        "backup_overall_status",
+        "backup_duration_seconds",
         "backup_last_run_timestamp",
-        "backup_size_gb",
-        "backup_duration_minutes",
+        "backup_total_services",
+        "backup_total_repo_size_gb",
+        "backup_services_completed_count",
+        "backup_services_failed_count",
+        "backup_services_disabled_count",
     ];
     
     /// Get all metric subscriptions for a widget type