Add service_type field to separate data from presentation

Changes:
- Add service_type field to SubServiceData: 'nginx_site', 'container', 'image'
- Agent sends pure data without display formatting
- Dashboard checks service_type to decide presentation
- Docker images now display without status icon (service_type='image')
- Remove unused image_size_str from docker images tuple

Clean separation: agent provides data, dashboard handles display logic.

shared/src/agent_data.rs

@@ -149,6 +149,9 @@ pub struct SubServiceData {
     pub name: String,
     pub service_status: Status,
     pub metrics: Vec<SubServiceMetric>,
+    /// Type of sub-service: "nginx_site", "container", "image"
+    #[serde(default)]
+    pub service_type: String,
 }
 
 /// Individual metric for a sub-service