feat(voice): 豆包 flash ASR，支持热词与对话上下文

用极速版 HTTP 替代 Whisper；默认热词「小荷」，可选 history/--correct。

README.md

@@ -56,7 +56,7 @@ mytoolkit --help
 | `latex` | `compile` / `count` |
 | `bib` | `to-markdown` / `cv-update` |
 | `video` | `avi-to-mp4` |
-| `voice` | Text-to-speech via Volcengine/Doubao (`voice tts`, mp3/wav/pcm) |
+| `voice` | Text-to-speech / speech-to-text via Volcengine/Doubao (`voice tts`, `voice stt`) |
 | `webpage` | Docusaurus build/list/rebuild/push |
 | `mail` | IMAP/SMTP client (read/draft/forward/reply, no auto-send) |
 | `templates` | Inspect template root; external override still possible |