Add formatHolderCount helper for compact holder display.

[Yormee-103]

Summary

This batch covers four contributor and UX improvements across docs, formatting, empty states, and test coverage:

Closes #439 — Added a Shared utility helpers section to CONTRIBUTING.md documenting address shortening, URL/handle/price formatting helpers with input/output examples, and a note to add new formatting logic in src/utils/ instead of inline in components.

Closes #441 — Added integration tests for Change24hBadge direction indicators (up/down/none for positive, negative, and zero change) and fixed zero-change behavior so it no longer shows the minus icon.

Closes #440 — Added EmptyCreatorList for zero-result creator searches with a readable message and Clear search button; wired it into LandingPage with component and landing-page tests.

Closes #438 — Added formatHolderCount() in numberFormat.utils.ts for compact holder counts (999, 1.2K, 2.4M) with unit tests for each range and boundary values at 1,000 and 1,000,000.
Test plan

Testing

• pnpm lint
• pnpm build

Checklist

• Linked issue or backlog item
• Scope is limited to the stated change
• Updated docs if behavior or setup changed
• Added screenshots for UI changes when relevant

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