Add a native Qt "Bigscreen" shell: CMakeLists, C++ entry (main.cpp, InputRouter), QML module (Theme, ShellWindow, views and components) and a Bigscreen/.gitignore; update top-level .gitignore and README with Qt build/run instructions. Remove the legacy Tauri/web prototype files (package.json, package-lock.json, src-tauri and many web assets) as part of the migration to the Qt/CMake-based shell.
