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/main.rs

@@ -6,10 +6,8 @@ use tracing_subscriber::EnvFilter;
 mod app;
 mod communication;
 mod config;
-mod hosts;
 mod metrics;
 mod ui;
-mod utils;
 
 use app::Dashboard;
 
@@ -22,7 +20,7 @@ struct Cli {
     #[arg(short, long, action = clap::ArgAction::Count)]
     verbose: u8,
 
-    /// Configuration file path
+    /// Configuration file path (required)
     #[arg(short, long)]
     config: Option<String>,