feat(desktop): window translucency slider in Appearance settings (#45086)

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.

apps/desktop/src/main.tsx

@@ -1,4 +1,6 @@
 import './styles.css'
+// Side-effect: applies the persisted window translucency on load.
+import './store/translucency'
 
 import { QueryClientProvider } from '@tanstack/react-query'
 import { StrictMode } from 'react'