Implement WakeOnLAN functionality with simplified configuration

- Add Status::Offline enum variant for disconnected hosts
- All configured hosts now always visible showing offline status when disconnected
- Add WakeOnLAN support using wake-on-lan Rust crate
- Implement w key binding to wake offline hosts with MAC addresses
- Simplify configuration to single [hosts] section with MAC addresses only
- Change critical status icon from ◯ to ! for better visibility
- Add proper MAC address parsing and error handling
- Silent WakeOnLAN operation with logging for success/failure

Configuration format:
[hosts]
hostname = { mac_address = "AA:BB:CC:DD:EE:FF" }

agent/src/collectors/backup.rs

@@ -140,6 +140,7 @@ impl Collector for BackupCollector {
                 Status::Warning => "warning".to_string(),
                 Status::Critical => "critical".to_string(),
                 Status::Unknown => "unknown".to_string(),
+                Status::Offline => "offline".to_string(),
             }),
             status: overall_status,
             timestamp,
@@ -202,6 +203,7 @@ impl Collector for BackupCollector {
                     Status::Warning => "warning".to_string(),
                     Status::Critical => "critical".to_string(),
                     Status::Unknown => "unknown".to_string(),
+                    Status::Offline => "offline".to_string(),
                 }),
                 status: service_status,
                 timestamp,