refactor: split core/ui/apps targets and add unit tests

Move all business logic into liblivecaption_core (no Qt dependency) and
the widgets into liblivecaption_ui, leaving two executables: the caption
window and the CLI, which no longer links Qt Widgets. Abstract the ASR
backend behind IAsr so the streaming policy and the live pipeline can be
tested with a scripted fake backend instead of a real model.

把全部业务逻辑下沉到不依赖 Qt 的 liblivecaption_core，界面独立成
liblivecaption_ui，拆出悬浮窗与命令行两个可执行程序（CLI 不再链接 Qt
Widgets）；并用 IAsr 抽象推理后端，使流式策略与实时管线可脱离真模型
用假实现做单元测试。测试还抓到一个真 bug：段恰好结束在 chunk 边界时
只提交了稳定部分，最后几个词被丢掉，已修。

src/ui/livetranscriber.cpp

@@ -0,0 +1,60 @@
+#include "livetranscriber.h"
+
+#include "iasr.h"
+
+namespace lc {
+
+LiveTranscriber::LiveTranscriber(IAsr & asr, QObject * parent)
+    : QObject(parent), asr_(asr)
+{
+}
+
+LiveTranscriber::~LiveTranscriber()
+{
+    stop();
+}
+
+QString LiveTranscriber::deviceName() const
+{
+    return pipeline_ ? QString::fromStdString(pipeline_->deviceName()) : QString();
+}
+
+bool LiveTranscriber::start(const StreamOptions & opts, QString * error)
+{
+    stop();
+
+    LivePipelineOptions popt;
+    popt.stream = opts;
+
+    pipeline_ = std::make_unique<LivePipeline>(asr_, popt);
+
+    // 回调在工作线程上被调用；连到主线程的接收者时 Qt 会自动排队投递。
+    pipeline_->setCaptionCallback([this](const std::string & committed,
+                                         const std::string & tail,
+                                         const std::string & language) {
+        Q_EMIT caption(QString::fromStdString(committed),
+                       QString::fromStdString(tail),
+                       QString::fromStdString(language));
+    });
+    pipeline_->setErrorCallback([this](const std::string & message) {
+        Q_EMIT failed(QString::fromStdString(message));
+    });
+
+    std::string err;
+    if (!pipeline_->start(&err)) {
+        if (error) *error = QString::fromStdString(err);
+        pipeline_.reset();
+        return false;
+    }
+    return true;
+}
+
+void LiveTranscriber::stop()
+{
+    if (pipeline_) {
+        pipeline_->stop();
+        pipeline_.reset();
+    }
+}
+
+} // namespace lc