feat: show real-time download progress bar for app updates

Leverage the Tauri updater plugin's DownloadEvent callback to display
a progress bar with downloaded/total bytes during app updates, replacing
the previous spinner-only feedback.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/i18n/messages/ko.json

@@ -172,6 +172,7 @@
     "checking": "확인 중...",
     "checkUpdate": "업데이트 확인",
     "updating": "설치 중...",
+    "downloading": "다운로드 중...",
     "upgradeTo": "v{version}로 업그레이드",
     "foundUpdate": "새 버전 v{version}을 찾았습니다",
     "alreadyLatest": "이미 최신 버전입니다",