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。
Add a second backend (sherpa-onnx) alongside llama.cpp, so native
streaming models can be selected by ID from a JSON registry. All such
models share one adapter: only the model files and a few options differ.
Models now live under the app data directory and are fetched by
scripts/fetch-models.sh. Adding a model is a data change, not code.
Measured on the same audio: the streaming backend is ~35x cheaper in CPU
than Qwen3-ASR (0.10-0.12 vs 4.27 core-seconds per audio second), shows
the first caption at 0.48-1.44s, and matches its accuracy for zh/en.
新增 sherpa-onnx 流式后端，与 llama.cpp 并存；模型按注册表 id 从应用
数据目录加载。同族模型共用一个适配器，新增模型只改 data/models.json。
实测流式后端 CPU 约为 Qwen3-ASR 的 1/35，首个字幕 0.48~1.44s，中英
准确率持平。注意预编译库是旧 C++ ABI，必须走其 C API。
blumia