Add notification reminder diagnostics to system health

[borg-codex-bot]

Summary

Add a read-only diagnostics view for scheduled notification reminders.

Problem

The notification-state file only records reminders after they were sent or cleaned up. It does not show upcoming overdue windows, why a reminder is not yet sent, or when the next reminder would become eligible. This makes testing backup_overdue and restore_test_overdue difficult.

Expected behavior

System health should expose a diagnostics block that shows active reminder checks for configured notification channels.

Scope

• Show backup overdue diagnostics when at least one notification channel has backup_overdue enabled.
• Show restore-test overdue diagnostics when at least one notification channel has restore_test_overdue enabled.
• Display job, expected/due time, overdue threshold, latest status/test, reminder state, and reason.
• Diagnostics must be read-only and must not send notifications or update notification-state.

User impact

Improves testability and supportability for scheduled notification reminders.

[borg-codex-bot]

Implemented and released in 2026.07.02.0951. Notification reminder diagnostics are now available under Settings > Advanced, separated from per-repository passphrases, with clearer backup and restore-test overdue state, next scheduled run handling, and lighter table styling.