In-app notification center with read/unread state

[joelpeace48-cell]

Epic: Notifications & Messaging

Summary

A persistent in-app inbox for reward/campaign events.

Background & Problem

An inbox ensures users don't miss important events even without email/push.

Acceptance Criteria

• Inbox lists events with read state.
• Real-time updates.
• Paginated.

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Filed as part of the mainnet-readiness & world-class roadmap. Scope is intentionally small enough for one focused PR; comment to claim.