Add a frameless always-on-top caption window wired to the live capture pipeline. Silence-based segmentation gates the audio so the model never sees silence, and the panel background is painted in paintEvent with a configurable opacity instead of a style sheet. 新增悬浮字幕窗(无边框、置顶、可折叠/展开),接入实时采集与流式转写。 分段由静音检测驱动,静音期间不送音频,避免模型对静音产生幻觉。底板 改为在 paintEvent 中绘制,不透明度可调。