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

agent/src/main.rs

@@ -11,7 +11,6 @@ mod config;
 mod metrics;
 mod notifications;
 mod status;
-mod utils;
 
 use agent::Agent;