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。

docs/testing.md

@@ -53,6 +53,7 @@ ctest --test-dir build --output-on-failure    # 也可以单独跑 build/tests/t
 | `tst_livepipeline.silenceEndsSegment` | 静音收束分段 |
 | `tst_vad.quietSpeechIsDetected` | 噪声底用均值跟踪，把很轻的话音全判成静音 |
 | `tst_transcript.*` | 输出解析与语言名映射 |
+| `tst_transcript.isoCodeDisplayName` / `registryLanguagesAllHaveDisplayNames` | 设置里的语言提示用的是 ISO 码（`zh`/`uk`…），漏了显示名的话「识别语言」下拉会直接露出 `uk` 这种码 |
 
 写测试的过程中还**抓到一个真 bug**：段恰好结束在 chunk 边界（没有待处理音频）时，`flush()` 只提交"已稳定"部分，最后几个词会被丢掉。已修（用完整文本 `lastRawFull_` 提交）。