Add API key support for git authentication

- Add nixos_config_api_key_file option to NixOS configuration
- Support reading API token from file for private repositories
- Automatically inject token into HTTPS URLs (https://token@host/repo.git)
- Graceful fallback to original URL if key file missing/empty
- Default key file location: /var/lib/cm-dashboard/git-api-key

Usage: echo 'your-api-token' | sudo tee /var/lib/cm-dashboard/git-api-key

dashboard/src/config/mod.rs

@@ -28,6 +28,7 @@ pub struct SystemConfig {
     pub nixos_config_git_url: String,
     pub nixos_config_branch: String,
     pub nixos_config_working_dir: String,
+    pub nixos_config_api_key_file: Option<String>,
 }
 
 impl DashboardConfig {