Fix zero amount rendering in activity feed

[eliolewis77]

Summary

• render activity amounts when amount is explicitly numeric, including 0
• replace the landing page duplicate activity markup with the shared ActivityList component
• add ActivityList coverage for zero, undefined, and positive amounts
• add ts-node as a dev dependency so the existing TypeScript Jest config can be loaded by npm test

Fixes #87

Behavior

I chose to render amount: 0 as formatted currency (0 USDC) because zero is a valid amount distinct from undefined, which remains the no-amount case.

Validation

• npm test -- --runInBand --no-watchman
• npm run lint
• git diff --check

NEXT_PUBLIC_STELLAR_NETWORK=TESTNET npm run build is still blocked on existing main issues unrelated to this PR: duplicate validateTeamSplits exports in src/lib/utils.ts, plus Google Fonts fetch failures in this sandboxed environment.

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