fix(site): audit PWA notification support across Edge and iOS

[DevSecNinja]

Problem

Current notification behavior is confusing in real browsers:

• Edge on Windows, installed as a PWA, can show Notifications unavailable.
• iOS Safari installed as a PWA appears to notify only when the app is opened.

That may partly reflect platform limitations, but the app should make the actual support state clearer and avoid incorrectly implying that hourly background notifications are active when the browser cannot provide them.

Current implementation

• The page asks for Notification permission.
• The service worker stores latest briefing state and can show notifications from sync/periodicsync events.
• The app falls back to open-app/reopen checks when background APIs are unavailable.

Scope

• Test installed PWA and normal browser tab behavior in Edge on Windows.
• Test iOS/iPadOS Safari installed web app behavior.
• Distinguish permission denied, Notification API unavailable, service-worker notification unavailable, and background sync unavailable states in the UI.
• Document which modes are expected to work and where.
• Improve button/status copy so users understand whether notifications are background-capable or only app-open checks.

Acceptance criteria

• Browser support notes for Edge Windows and iOS/iPadOS PWA.
• UI copy or state handling improved where current Notifications unavailable is misleading.
• Tests updated for any changed notification state text or feature detection.