fix: guard localStorage SSR in NotificationCenterProvider and suppress hydration mismatches

[Joyful12-tech]

Summary

Fixes the SSR hydration issues reported in #290.

Root causes

1. NotificationCenterProvider — window.localStorage was accessed only inside useEffect (correct), but the unreadCount derived from the pre-hydration state was used by NotificationBell to render a badge, causing a hydration mismatch.
2. NotificationBell — rendered the unread badge count from context before localStorage was read, so the server HTML (count = 3) differed from the initial client render.
3. NotificationFeed — called formatRelativeTime() (which calls Date.now()) during render; the server timestamp and the client timestamp differ slightly, triggering React's hydration warning.
4. notifications.ts — formatRelativeTime had no fallback if Intl.RelativeTimeFormat is unavailable and no documentation on the SSR hydration caveat.

Changes

File	Change
NotificationCenterProvider.tsx	Guard localStorage with typeof window !== 'undefined'; add try/catch on persist write; expose hasHydrated in context
NotificationBell.tsx	Use displayCount = hasHydrated ? unreadCount : 0 so the badge is always 0 on the server render, matching the initial client render
NotificationFeed.tsx	Add suppressHydrationWarning on the <span> containing formatRelativeTime() output
notifications.ts	Document pure functions; add try/catch fallback in formatRelativeTime; add JSDoc explaining hydration caveat
vitest.config.ts	New — configures jsdom environment and @/* alias for Vitest
vitest.setup.ts	New — imports @testing-library/jest-dom matchers
src/__tests__/notifications.test.tsx	New — 23 test cases covering sortNotifications, filterNotifications, formatRelativeTime, and NotificationCenterProvider SSR hydration

Testing

# from app/frontend
pnpm vitest run

All 23 cases pass.

Closes #290

[MaryammAli]

LGTM