[codex] build role-based activity feed

[prissca]

Summary

• add a reusable persisted activity service and authenticated activity API
• provide role-scoped investor, driver, and admin timeline pages
• support wallet, investment, repayment, KYC, vehicle, payout, Stellar, and system categories
• persist individual and bulk read state and synchronize unread counts with dashboard headers
• add loading, error, empty, filtering, and deep-link states
• redirect the legacy driver notifications page to the new activity feed

Why

Dashboard notifications were split between a persisted notification model and hardcoded client-only UI. Users could not reliably review role-relevant events or preserve read state between sessions.

Validation

• npm run lint (passes; three pre-existing warnings remain)
• NEXT_PUBLIC_PRIVY_APP_ID=cl00000000000000000000000 npm run build (passes; a 25-character build placeholder is needed because .env.example intentionally contains an invalid Privy ID)
• targeted ESLint over all changed TypeScript files (passes)

Closes #49

[Obiajulu-gif]

@prissca great work