Remove hardcoded defaults and migrate dashboard config to NixOS

- Remove all unused configuration options from dashboard config module
- Eliminate hardcoded defaults - dashboard now requires config file like agent
- Keep only actually used config: zmq.subscriber_ports and hosts.predefined_hosts
- Remove unused get_host_metrics function from metric store
- Clean up missing module imports (hosts, utils)
- Make dashboard fail fast if no configuration provided
- Align dashboard config approach with agent configuration pattern

dashboard/src/app.rs

@@ -28,11 +28,14 @@ impl Dashboard {
     pub async fn new(config_path: Option<String>, headless: bool) -> Result<Self> {
         info!("Initializing dashboard");
 
-        // Load configuration
-        let config = if let Some(path) = config_path {
-            DashboardConfig::load_from_file(&path)?
-        } else {
-            DashboardConfig::default()
+        // Load configuration - config file is required
+        let config = match config_path {
+            Some(path) => DashboardConfig::load_from_file(&path)?,
+            None => {
+                error!("Configuration file is required. Use --config to specify path.");
+                error!("Dashboard configuration must be provided - no hardcoded defaults allowed.");
+                return Err(anyhow::anyhow!("Missing required configuration file"));
+            }
         };
 
         // Initialize ZMQ consumer