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/store/translucency.ts

@@ -0,0 +1,38 @@
+/**
+ * Window translucency (see-through window).
+ *
+ * One lever, 0–100. 0 = off (fully opaque, the default). Higher = more of the
+ * desktop shows through the whole window — the main process maps it to the
+ * native window opacity (`setOpacity`), the same effect as the Windows
+ * shift-scroll trick. macOS + Windows only; Linux has no runtime window
+ * opacity, so it's a no-op there.
+ *
+ * The renderer owns the value and mirrors it to the main process over IPC.
+ */
+
+import { atom } from 'nanostores'
+
+import { persistString, storedString } from '@/lib/storage'
+
+const KEY = 'hermes.desktop.translucency.v1'
+
+const clamp = (n: number): number => Math.min(100, Math.max(0, Math.round(n)))
+
+const read = (): number => {
+  const n = Number(storedString(KEY))
+
+  return Number.isFinite(n) ? clamp(n) : 0
+}
+
+export const $translucency = atom<number>(typeof window === 'undefined' ? 0 : read())
+
+export function setTranslucency(intensity: number): void {
+  $translucency.set(clamp(intensity))
+}
+
+if (typeof window !== 'undefined') {
+  $translucency.subscribe(intensity => {
+    persistString(KEY, String(intensity))
+    window.hermesDesktop?.setTranslucency?.({ intensity })
+  })
+}