Fix notification time and history contracts

[hamzamerzic]

Summary

• restore UTC offsets on notification timestamps at the response boundary
• use a stable timestamp-and-id cursor and reject invalid cursors
• refresh relative time labels while the view stays open
• accept same-origin absolute notification targets without weakening target validation

Testing

• pytest tests/test_notifications_unread.py
• notification model and service-worker target tests
• production frontend build

[hamzamerzic]

Closing as fully superseded: after refreshing this branch onto current main, GitHub reports a zero-file diff and the branch tree is byte-identical to main. The reviewed notification-time/history work is already present through #304 (with the navigation/offline follow-up in #326), so merging this history-only duplicate would add no code.