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/ja.json

@@ -172,6 +172,7 @@
     "checking": "確認中...",
     "checkUpdate": "更新を確認",
     "updating": "インストール中...",
+    "downloading": "ダウンロード中...",
     "upgradeTo": "v{version} にアップグレード",
     "foundUpdate": "新しいバージョン v{version} が見つかりました",
     "alreadyLatest": "すでに最新バージョンです",