Replace device names with serial numbers in MergerFS pool display

Updates disk collector and dashboard to show drive serial numbers
instead of device names (sdX) for MergerFS data/parity drives.
Agent extracts serial numbers from SMART data and dashboard
displays them when available, falling back to device names.

shared/src/agent_data.rs

@@ -104,6 +104,7 @@ pub struct PoolData {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct PoolDriveData {
     pub name: String,
+    pub serial_number: Option<String>,
     pub temperature_celsius: Option<f32>,
     pub wear_percent: Option<f32>,
     pub health: String,