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/electron/preload.cjs

@@ -40,6 +40,7 @@ contextBridge.exposeInMainWorld('hermesDesktop', {
   stopPreviewFileWatch: id => ipcRenderer.invoke('hermes:stopPreviewFileWatch', id),
   setTitleBarTheme: payload => ipcRenderer.send('hermes:titlebar-theme', payload),
   setNativeTheme: mode => ipcRenderer.send('hermes:native-theme', mode),
+  setTranslucency: payload => ipcRenderer.send('hermes:translucency', payload),
   setPreviewShortcutActive: active => ipcRenderer.send('hermes:previewShortcutActive', Boolean(active)),
   openExternal: url => ipcRenderer.invoke('hermes:openExternal', url),
   fetchLinkTitle: url => ipcRenderer.invoke('hermes:fetchLinkTitle', url),