Add automatic config file detection for dashboard TUI

- Dashboard now automatically looks for /etc/cm-dashboard/dashboard.toml
- No need to specify --config flag when using standard NixOS deployment
- Fallback to manual config path if default not found
- Update help text to reflect optional config parameter
- Simplifies dashboard usage - just run 'cm-dashboard' without arguments

dashboard/src/main.rs

@@ -20,7 +20,7 @@ struct Cli {
     #[arg(short, long, action = clap::ArgAction::Count)]
     verbose: u8,
 
-    /// Configuration file path (required)
+    /// Configuration file path (defaults to /etc/cm-dashboard/dashboard.toml)
     #[arg(short, long)]
     config: Option<String>,