chore(pack): add CPack configuration to build a deb package

Add install rules and CPack settings so cpack produces an installable
.deb. Dependencies are derived with dpkg-shlibdeps, so they follow the
binaries. The sherpa-onnx runtime the streaming backend needs is bundled
because no distro package provides it. The fetch-models script now also
locates its registry when installed system-wide.

增加 CPack 配置，可直接用 cpack 打出 deb：安装规则含桌面入口、应用图标、
模型注册表与下载脚本、用户文档；依赖由 dpkg-shlibdeps 自动推导；流式后端
需要的 sherpa-onnx 运行时（发行版无对应包）随包携带；fetch-models 脚本
装到系统后也能找到注册表。

data/live-caption.desktop

@@ -0,0 +1,17 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=Live Caption
+Name[zh_CN]=实时字幕
+GenericName=Real-time captions
+GenericName[zh_CN]=实时字幕
+Comment=Show live subtitles for the audio your system is playing
+Comment[zh_CN]=把系统正在播放的声音实时转写成字幕
+Exec=live-caption
+TryExec=live-caption
+Icon=live-caption
+Terminal=false
+Categories=AudioVideo;Audio;
+Keywords=caption;subtitle;subtitles;speech;transcription;asr;
+Keywords[zh_CN]=字幕;实时字幕;语音识别;听写;
+StartupNotify=false