A see-through-window control (0–100, off by default) that maps to the
native window opacity via setOpacity — the desktop shows through the whole
window, the same effect as the Windows shift-scroll trick. macOS + Windows;
a no-op on Linux (no runtime window opacity).
Renderer owns the value (persisted, nanostore) and mirrors it to the main
process over IPC; main persists it to translucency.json so a cold launch
applies it at window creation before the renderer reports in.
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