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。

tests/tst_transcript.cpp

@@ -42,6 +42,40 @@ private slots:
         QCOMPARE(QString::fromStdString(lc::languageDisplayName("Klingon")), QString("Klingon"));
         QVERIFY(lc::languageDisplayName("").empty());
     }
+
+    // 设置里的语言提示是 ISO 码，界面要把它显示成中文名。
+    // 覆盖注册表里出现过的所有语言码，避免加了模型却漏了显示名。
+    void isoCodeDisplayName()
+    {
+        const struct { const char * code; const char * display; } kCases[] = {
+            { "zh", "中文" },   { "en", "英语" },     { "ja", "日语" },
+            { "ko", "韩语" },   { "fr", "法语" },     { "de", "德语" },
+            { "es", "西班牙语" },{ "ru", "俄语" },    { "ar", "阿拉伯语" },
+            { "hi", "印地语" }, { "vi", "越南语" },   { "it", "意大利语" },
+            { "pt", "葡萄牙语" },{ "nl", "荷兰语" },  { "tr", "土耳其语" },
+            { "uk", "乌克兰语" },{ "auto", "自动检测" },
+        };
+        for (const auto & c : kCases) {
+            QCOMPARE(QString::fromStdString(lc::languageDisplayNameFromCode(c.code)),
+                     QString::fromUtf8(c.display));
+        }
+        // 未知码原样返回；空串返回空
+        QCOMPARE(QString::fromStdString(lc::languageDisplayNameFromCode("xx")), QString("xx"));
+        QVERIFY(lc::languageDisplayNameFromCode("").empty());
+    }
+
+    // 注册表里每个模型声明的语言都必须能显示成中文名，
+    // 否则设置里的「识别语言」下拉会出现英文码。
+    void registryLanguagesAllHaveDisplayNames()
+    {
+        for (const char * code : { "zh", "en", "ja", "ko", "fr", "de", "es", "ru", "ar",
+                                   "hi", "vi", "it", "pt", "nl", "tr", "uk" }) {
+            const QString display =
+                QString::fromStdString(lc::languageDisplayNameFromCode(code));
+            QVERIFY2(display != QString::fromLatin1(code),
+                     qPrintable(QStringLiteral("语言码没有显示名: %1").arg(code)));
+        }
+    }
 };
 
 QTEST_MAIN(TstTranscript)