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/zh-TW.json

@@ -172,6 +172,7 @@
     "checking": "檢查中...",
     "checkUpdate": "檢查更新",
     "updating": "升級中...",
+    "downloading": "下載中...",
     "upgradeTo": "升級到 v{version}",
     "foundUpdate": "發現新版本 v{version}",
     "alreadyLatest": "目前已是最新版本",