feat: stage online upgrades before verified restarts

Download updates asynchronously with persistent progress and validate matching source and build artifacts before an explicit restart. Keep upgrade workers alive across service restarts, verify the new runtime, and recover the browser with a one-time success notice.

Publish immutable archive sizes and checksums for accurate download progress. Align the update dialog, hide unavailable actions during transfers, and show the sidebar notice only for active, ready or failed jobs. Constrain long account names with ellipsis while preserving avatar and collapse controls.

back/shared/i18n.ts

@@ -1,6 +1,8 @@
 const messages: Record<string, Record<string, string>> = {
   zh: {},
   en: {
+    '升级任务启动失败，请检查日志后重试':
+      'Failed to start the update. Check the logs and try again.',
     '[schedule][停止已运行任务] 任务ID: %s, PID: %s':
       '[schedule][Stopping running task] Task ID: %s, PID: %s',