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

agent/src/collectors/mod.rs

@@ -7,6 +7,7 @@ pub mod cpu;
 pub mod memory;
 pub mod disk;
 pub mod systemd;
+pub mod backup;
 pub mod error;
 
 pub use error::CollectorError;