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" }

shared/src/metrics.rs

@@ -87,6 +87,7 @@ pub enum Status {
     Warning,
     Critical,
     Unknown,
+    Offline,
 }
 
 impl Status {
@@ -190,6 +191,16 @@ impl HysteresisThresholds {
                     Status::Ok
                 }
             }
+            Status::Offline => {
+                // Host coming back online, use normal thresholds like first measurement
+                if value >= self.critical_high {
+                    Status::Critical
+                } else if value >= self.warning_high {
+                    Status::Warning
+                } else {
+                    Status::Ok
+                }
+            }
         }
     }
 }