Add service metrics from systemctl (memory, uptime, restarts)

Shared:
- Add memory_bytes, restart_count, uptime_seconds to ServiceData

Agent:
- Add new fields to ServiceStatusInfo struct
- Fetch MemoryCurrent, NRestarts, ExecMainStartTimestamp from systemctl show
- Calculate uptime from start timestamp
- Parse and populate new fields in ServiceData
- Remove unused load_state and sub_state fields

Dashboard:
- Add memory_bytes, restart_count, uptime_seconds to ServiceInfo
- Update header: Service, Status, RAM, Uptime, ↻ (restarts)
- Format memory as MB/GB
- Format uptime as Xd Xh, Xh Xm, or Xm
- Show restart count with ! prefix if > 0 to indicate instability

All metrics obtained from single systemctl show call - zero overhead.

shared/src/agent_data.rs

@@ -139,6 +139,12 @@ pub struct ServiceData {
     pub user_stopped: bool,
     pub service_status: Status,
     pub sub_services: Vec<SubServiceData>,
+    /// Memory usage in bytes (from MemoryCurrent)
+    pub memory_bytes: Option<u64>,
+    /// Number of service restarts (from NRestarts)
+    pub restart_count: Option<u32>,
+    /// Uptime in seconds (calculated from ExecMainStartTimestamp)
+    pub uptime_seconds: Option<u64>,
 }
 
 /// Sub-service data (nginx sites, docker containers, etc.)