feat: add configurable key bindings with ESC quit disable option

- Add unified application configuration system with JSON config file
- Implement configurable quit keys (default: "q", "ctrl+c")
- Add disable_esc_quit option for vim users to prevent accidental exits
- Auto-create default config file at ~/.config/sshm/config.json
- Maintain backward compatibility (ESC quit enabled by default)
- Include comprehensive tests and documentation

internal/ui/model.go

@@ -80,6 +80,9 @@ type Model struct {
 	sortMode       SortMode
 	configFile     string // Path to the SSH config file
 
+	// Application configuration
+	appConfig      *config.AppConfig
+
 	// Version update information
 	updateInfo     *version.UpdateInfo
 	currentVersion string