[sunred-liu]

closes #1112

Summary

Adds an accessible delivery preview to the notification preferences screen (issue #1112) so an owner can see exactly what they will receive before saving — the channels a message travels on, and the precise event categories that will trigger delivery.

What changed (all additive; the panel only gains a preview section)

• src/utils/notificationPreview.ts — pure, dependency-free helper buildNotificationPreview(prefs) that derives, from the current selections: enabled channels (in-app is always available; sound / vibration / browser push reflect the toggles), the list of event categories that will trigger delivery, a privacy-mode note when Do Not Disturb quiet hours are on, and an isEmpty flag when no event type is enabled.
• src/components/Notifications/NotificationPreferencePreview.tsx — a keyboard- and screen-reader-friendly panel (<section> + real <ul> lists, a live role="status" summary, status text that does not rely on colour alone) rendered above the Save button, updating live as the owner toggles channels and categories.
• src/components/Notifications/NotificationPreferencesPanel.tsx — mounts the preview; no existing toggle or behaviour was changed.
• Unit tests: notificationPreview.test.ts (channel/category derivation, DND quiet-hours summary, empty/denied state, single-category boundary) and NotificationPreferencePreview.test.tsx (render + jest-axe accessibility, no violations).

Privacy & scope

• Authorization and consent remain server-side: this PR only reflects the owner's local selections and never sends or enforces anything. It surfaces what the backend will do once saved.
• No PII is shown; channel samples are illustrative text only.

Verification

• jest (ts-jest): 16/16 new tests passing across the two suites; full suite failure count unchanged vs baseline (104 pre-existing, environment-related) — zero regression.
• tsc --noEmit: no type errors in the added/changed files.