feat(notification-bell): wrap icon button with accessible Tooltip primitive (Closes #763)

[waterWang]

Description

Wraps NotificationBell's icon button with the shared Tooltip primitive to provide a hover/touch tooltip describing the notification state, as requested in issue #763.

Changes

• src/pages/NotificationBell.tsx: Wrapped both the reduced-motion (static) and full-motion (animated) button branches with <Tooltip content={ariaLabel} placement="bottom">. The tooltip shows the same accessible label text (e.g. "Notifications — 5 unread" or "Notifications") on hover, focus, or long-press.
• src/pages/NotificationBell.tooltip.test.tsx: 2 tests covering:
  • Tooltip trigger with unread count (accessible label match)
  • Tooltip trigger without unread notifications

Testing

• ✅ npx jest src/pages/NotificationBell.tooltip — 2/2 passing
• ✅ TypeScript compilation — no errors
• ✅ Existing NotificationBell tests unaffected

Closes #763

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