feat: add a mobile app section and refresh the hero copy

[letom1176-spec]

What & why

The landing page doesn't mention Gibwork's mobile app and reads as a generic freelance marketplace rather than the on-chain, pay-to-your-wallet bounty market it is. This adds a dedicated mobile-app section and refreshes the hero copy to match the product.

Changes

• New MobileApp section (components/mobile-app.tsx), rendered right after the hero — two columns on desktop, stacked on mobile, using the repo's existing framer-motion + Tailwind + shadcn patterns and design tokens. It has a phone mockup built from in-repo assets only (no new images) and an Android store badge.
• Store links centralized in lib/site-config.ts: androidUrl points to the live Google Play listing (com.gibwork.app); iosUrl is left empty with a TODO, and the App Store badge renders conditionally so it stays hidden until a real iOS URL exists.
• Brand marks added as SVG components under components/logo/ (apple.tsx, googleplay.tsx) to match the existing convention. No new dependencies.
• Hero sub-headline refreshed to reflect the on-chain product.

Notes

• No new dependencies.
• Happy to adjust the copy, placement, or add the iOS link if there's a public App Store listing.

[vercel]

@letom1176-spec is attempting to deploy a commit to the gibwork Team on Vercel.

A member of the Team first needs to authorize it.