Fixes duplicate/triple OS notifications on machines where more than one browser (e.g. both Chrome and Edge on Windows) independently got notification permission for the site — each held its own valid subscription, so every event pushed to both. Adds a userAgent column (captured client-side at subscribe time, display-only) and a Settings > Account section listing a user's own subscribed browsers with a friendly label plus an unsubscribe button, backed by new GET/DELETE /api/push/subscriptions endpoints scoped to the caller's own rows. Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01Gh2UXUQUVBWroWEnn1FLFG