feat(ui): add floating caption window with live transcription

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。

CMakeLists.txt

@@ -25,10 +25,12 @@ foreach(_p
     endif()
 endforeach()
 
-find_package(Qt6 REQUIRED COMPONENTS Core)
+find_package(Qt6 REQUIRED COMPONENTS Core Widgets)
 find_package(PkgConfig REQUIRED)
 find_package(Threads REQUIRED)
 
+set(CMAKE_AUTOMOC ON)
+
 pkg_check_modules(SAMPLERATE REQUIRED IMPORTED_TARGET samplerate)
 pkg_check_modules(PULSE      REQUIRED IMPORTED_TARGET libpulse)
 
@@ -67,12 +69,15 @@ add_executable(live-caption-cli
     src/qwen3asr.cpp
     src/streamingasr.cpp
     src/audio.cpp
+    src/livetranscriber.cpp
+    src/captionwindow.cpp
 )
 
 target_include_directories(live-caption-cli PRIVATE src)
 
 target_link_libraries(live-caption-cli PRIVATE
     Qt6::Core
+    Qt6::Widgets
     Threads::Threads
     PkgConfig::SAMPLERATE
     PkgConfig::PULSE