feat(desktop): macOS vibrancy for the left sidebar with a toggle

Add native macOS vibrancy behind the left sidebar (the only translucent
surface; header/chat/right sidebar stay opaque), plus a setting to turn it off.

- Window created with vibrancy applied after first show (avoids the cold-launch
  no-composite quirk); minimize/restore suppress the frost during the genie
  animation. Renderer frosts the sidebar via --sidebar-vibrancy-overlay once
  data-oc-vibrancy[-ready] are set; project-actions pill matches when open.
- data-oc-vibrancy-ready defaults are set in cssGenerator (DOM guaranteed),
  not the preload (document-start race left the sidebar un-frosted on launch).
- prefers-reduced-transparency falls back to solid surfaces.
- Appearance settings (macOS desktop only): a checkbox to enable/disable
  vibrancy, persisted to settings.json and applied via a Save & restart button
  (vibrancy is a window-creation option, so it needs a relaunch).

packages/ui/src/lib/i18n/messages/es.settings.ts

@@ -1539,6 +1539,10 @@ export const settingsDict = {
   "settings.openchamber.visual.field.selectTimeFormatAria": "Seleccionar formato de hora",
   "settings.openchamber.visual.field.selectWeekStartAria": "Seleccionar inicio de la semana",
   "settings.openchamber.visual.actions.reloadThemes": "Recargar temas",
+  "settings.openchamber.visual.field.macVibrancy": "Transparencia de la ventana",
+  "settings.openchamber.visual.field.macVibrancyHint": "Usa el desenfoque nativo de macOS (vibrancy) detrás de la barra lateral. Desactívalo para superficies totalmente sólidas y opacas.",
+  "settings.openchamber.visual.actions.saveAndRestart": "Guardar y reiniciar",
+  "settings.openchamber.visual.actions.restarting": "Reiniciando…",
   "settings.openchamber.visual.actions.reloadingThemes": "Recargando temas...",
   "settings.openchamber.visual.field.themeImportInfoAria": "Información de importación de temas",
   "settings.openchamber.visual.field.themeImportInfoTooltip": "Importar temas personalizados desde ~/.config/openchamber/themes/",