refactor: move backend options into backend_config section

- Add BackendConfig map[string]any to Config struct
- Deprecate top-level output_path (backward compatible via EffectiveBackendConfig())
- NewHyprland now accepts opts map[string]any instead of raw outputPath
- Factory signature unified to func(*slog.Logger, map[string]any) for all backends
- Update README with backend_config docs and fix Sway skeleton signature
- Update example.yaml to show backend_config instead of output_path

internal/config/config.go

@@ -52,9 +52,16 @@ type Config struct {
 
 	// OutputPath overrides the generated monitor config file path.
 	// Supports ~ for home directory expansion.
+	// Deprecated: use backend_config.output_path instead.
 	// Empty means the default: ~/.config/hypr/monitors.conf.
 	OutputPath string `yaml:"output_path"`
 
+	// BackendConfig holds backend-specific configuration options.
+	// Keys and values vary by the selected backend.
+	// Hyprland supports:
+	//   output_path — path to the generated monitor config file
+	BackendConfig map[string]any `yaml:"backend_config"`
+
 	// External lists monitor identifiers that trigger docked mode.
 	// Each entry is a name (DP-1) or a desc: prefix (desc:Dell U2723QE).
 	External []string `yaml:"external"`
@@ -158,6 +165,25 @@ func (c *Config) validate() error {
 	return nil
 }
 
+// EffectiveBackendConfig merges top-level deprecated backend keys
+// (OutputPath) into the BackendConfig map. backend_config values
+// take priority over deprecated top-level keys.
+//
+// The caller receives a non-nil map even when no backend_config is
+// set and no deprecated keys are present.
+func (c *Config) EffectiveBackendConfig() map[string]any {
+	bc := c.BackendConfig
+	if bc == nil {
+		bc = make(map[string]any)
+	}
+	if c.OutputPath != "" {
+		if _, exists := bc["output_path"]; !exists {
+			bc["output_path"] = c.OutputPath
+		}
+	}
+	return bc
+}
+
 // countEnabled returns how many MonitorEntries are enabled.
 func countEnabled(entries []MonitorEntry) int {
 	n := 0