Refresh landing page positioning and mobile workflow

[caijintaoduolacmeng-star]

Summary

• refresh the hero positioning around crypto-native bounties and USDC-funded work
• add direct CTAs for browsing bounties, creating work, GitHub, and mobile app downloads
• replace generic partner copy with Gibwork-specific sponsor/contributor messaging
• add a mobile workflow section with app download CTAs and sponsor/earner steps

This addresses the Gibwork task asking for landing page updates around the mobile app, clearer Web3 positioning, and better user flow.

Verification

• npm run build
• git diff --check

Notes:

• Used the iOS App Store listing for the App Store CTA.
• Used the Android package visible from the Gibwork app bundle (com.gibwork.app) for the Google Play CTA.

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