feat(bar): 新增 tray_output_index 配置用于选择 tray 挂载的屏幕

include/zdwm/bar.h

@@ -39,6 +39,8 @@ typedef struct zdwm_bar_config_t {
   const char *binding_mode_bg;
   const char *binding_mode_fg;
 
+  int32_t tray_output_index;
+
   uint32_t window_padding_x;
   const char *window_bg;
   const char *window_fg;