Refactor services widget with unified system metrics display

- Rename alerts widget to hosts widget for clarity
- Add sub_service field to ServiceInfo for display differentiation
- Integrate system metrics (CPU load, memory, temperature, disk) as service rows
- Convert nginx sites to individual sub-service rows with tree structure
- Remove nginx site checkmarks - status now shown via row indicators
- Update dashboard layout to display system and service data together
- Maintain description lines for connection counts and service details

Services widget now shows:
- System metrics as regular service rows with status
- Nginx sites as sub-services with ├─/└─ tree formatting
- Regular services with full resource data and descriptions
- Unified status indication across all row types

dashboard/src/data/metrics.rs

@@ -116,6 +116,8 @@ pub struct ServiceInfo {
     pub disk_used_gb: f32,
     #[serde(default)]
     pub description: Option<Vec<String>>,
+    #[serde(default)]
+    pub sub_service: bool,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]