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/ko.settings.ts

@@ -1539,6 +1539,10 @@ export const settingsDict = {
   'settings.openchamber.visual.field.selectTimeFormatAria': '시간 형식 선택',
   'settings.openchamber.visual.field.selectWeekStartAria': '주 시작 요일 선택',
   'settings.openchamber.visual.actions.reloadThemes': '테마 다시 로드',
+  'settings.openchamber.visual.field.macVibrancy': '창 투명도',
+  'settings.openchamber.visual.field.macVibrancyHint': '사이드바 뒤에 macOS 기본 블러(vibrancy)를 사용합니다. 완전히 불투명한 표면을 원하면 끄세요.',
+  'settings.openchamber.visual.actions.saveAndRestart': '저장 후 재시작',
+  'settings.openchamber.visual.actions.restarting': '재시작 중…',
   'settings.openchamber.visual.actions.reloadingThemes': '테마 다시 로드 중...',
   'settings.openchamber.visual.field.themeImportInfoAria': '테마 가져오기 정보',
   'settings.openchamber.visual.field.themeImportInfoTooltip': '~/.config/openchamber/themes/에서 사용자 정의 테마를 가져옵니다',