fix(gui): drop a dead setting, make the language list and tail toggle real

Three settings were only half-wired:

- autoPickByLanguage was saved but never read anywhere: delete it.
- showUnstableTail had a checkbox and a setter, but nothing ever called
  the setter, so the checkbox did nothing: apply it in applyAppearance
  and re-render from the cached text instead of blanking the caption.
- the language dropdown was hard-coded to auto/zh/en/ja while the registry
  declares up to 16 languages per model: build it from the selected
  model's own language list, with an ISO-code to display-name mapping
  (plus a unit test covering every code in the registry). The caption
  window's language pill now follows the setting too, instead of keeping
  whatever it showed at startup.

windowPos is not dead -- it stores the dragged window position -- so it stays.

三个「半死」设置：autoPickByLanguage 存了但从没被读取，直接删掉；showUnstableTail
有复选框也有 setter，但从来没人调用 setter（复选框点了没用），改为在 applyAppearance
里生效并用缓存文本重画；「识别语言」下拉原来硬编码 自动/中/英/日，而注册表里每个模型
声明了最多 16 种语言，改为按所选模型的语言动态生成，并补 ISO 码→显示名映射与单元测试。
字幕窗的语言标签也跟着设置走了（原来只在启动时读一次）。windowPos 不是死设置（它存的
是拖动后的位置），保留。

apps/gui/main.cpp

@@ -50,12 +50,13 @@ QIcon makeTrayIcon()
     return QIcon(pm);
 }
 
-// 把设置应用到字幕窗（字号/不透明度/行数/位置）
+// 把设置应用到字幕窗（字号/不透明度/行数/位置/灰色尾部）
 void applyAppearance(lc::CaptionWindow & window, const lc::AppSettings & cfg)
 {
     window.setPanelOpacity(cfg.opacityPercent);
     window.setTextFontSize(cfg.fontSize);
     window.setLineCounts(cfg.collapsedLines, cfg.expandedLines);
+    window.setShowTail(cfg.showUnstableTail);
     if (cfg.hasWindowPos) window.move(cfg.windowPos);
 }
 
@@ -193,7 +194,8 @@ int main(int argc, char ** argv)
     applyAppearance(window, cfg);
     // 语言 pill：流式模型不提供语言识别，就显示设置里选的语言
     if (!cfg.language.isEmpty()) {
-        window.setLanguage(QString::fromStdString(lc::languageDisplayName(cfg.language.toStdString())));
+        window.setLanguage(
+            QString::fromStdString(lc::languageDisplayNameFromCode(cfg.language.toStdString())));
     }
 
     // 注意声明顺序：asr 必须在 live 之前声明，
@@ -259,24 +261,31 @@ int main(int argc, char ** argv)
     QString activeKey = backendKeyOf(cfg);
 
     const auto openSettings = [&] {
+        // 后端相关项变化时：先把语言标签跟设置对齐（流式模型没有自动检测结果来更新它），
+        // 再重启识别后端。
+        const auto applyBackend = [&]() -> bool {
+            if (backendKeyOf(cfg) == activeKey) return true;
+            window.setLanguage(cfg.language.isEmpty()
+                                   ? QString()
+                                   : QString::fromStdString(
+                                         lc::languageDisplayNameFromCode(cfg.language.toStdString())));
+            if (!startTranscriber()) return false;
+            activeKey = backendKeyOf(cfg);
+            return true;
+        };
+
         lc::SettingsDialog dlg(false);
         QObject::connect(&dlg, &lc::SettingsDialog::settingsApplied, &window, [&] {
             lc::models::ModelStore::setRootOverride(cfg.modelDir);
             applyAppearance(window, cfg);          // 透明度/字号等立刻可见
-            if (backendKeyOf(cfg) != activeKey) {
-                if (startTranscriber()) activeKey = backendKeyOf(cfg);
-            }
+            applyBackend();
         });
         if (dlg.exec() == QDialog::Accepted) {
             lc::models::ModelStore::setRootOverride(cfg.modelDir);
             applyAppearance(window, cfg);
-            if (backendKeyOf(cfg) != activeKey) {
-                if (!startTranscriber()) {
-                    QMessageBox::warning(&window, QObject::tr("启动失败"),
-                                         QObject::tr("模型加载失败，请检查设置。"));
-                } else {
-                    activeKey = backendKeyOf(cfg);
-                }
+            if (!applyBackend()) {
+                QMessageBox::warning(&window, QObject::tr("启动失败"),
+                                     QObject::tr("模型加载失败，请检查设置。"));
             }
         }
     };