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。
