docs(cli): show option default values in --help

Qt does not print default values in --help output; the default passed to
the option constructor is only a fallback for value(). Append them to
the descriptions instead, via one shared helper used by both programs.

Qt 不会把默认值印进 --help（构造参数里的默认值只作为 value() 的兜底），
改为在说明后拼上「（默认 X）」；两个程序共用同一处定义，并统一了措辞。

apps/gui/main.cpp

@@ -84,9 +84,9 @@ int main(int argc, char ** argv)
     parser.addHelpOption();
     lc::app::addModelOptions(parser);
     lc::app::addStreamOptions(parser);
-    parser.addOption(QCommandLineOption("opacity", "字幕背景不透明度（20-100，百分比）", "n", "84"));
-    parser.addOption(QCommandLineOption("screenshot", "启动后自动截图并退出（验证渲染用）", "path"));
-    parser.addOption(QCommandLineOption("screenshot-delay", "截图前等待秒数", "sec", "6"));
+    parser.addOption(lc::app::makeOption("opacity", "字幕背景不透明度，取值 20-100", "n", "84"));
+    parser.addOption(lc::app::makeOption("screenshot", "启动后自动截图并退出（验证渲染用）", "path"));
+    parser.addOption(lc::app::makeOption("screenshot-delay", "截图前等待秒数", "sec", "6"));
     parser.process(app);
 
     const lc::AsrOptions asrOpt = lc::app::asrOptions(parser);