Remove native window decorations and add transparent background with
rounded corners, border, and drop shadow. Add custom close button
(Linux) and minimize/maximize/close (Windows) in the header. Add
programmatic window dragging via mousedown and double-click to maximize.
Install tauri-plugin-os for platform detection. Move sync spinner to
bottom-right corner. Convert drawer layout from vw to cqi units to
support the padding-based shadow approach.