fix: point landing CTAs to canonical dapp

[snkk2x-collab]

Summary

• default landing CTA links to https://dapp-zcore.vercel.app when NEXT_PUBLIC_DAPP_URL is not set
• update .env.example, README, and CONTRIBUTING to document the canonical dapp URL
• keep all CTA components routed through getDappUrl()

Validation

• rg "app\\.zcore\\.xyz|zcore-xi|localhost:3001"
• pnpm build

Relates to Zcorehub/ZCore-dev#22.

[vercel]

@snkk2x-collab is attempting to deploy a commit to the Cbiux's projects Team on Vercel.

A member of the Team first needs to authorize it.

[snkk2x-collab]

Local validation passed with pnpm build. The current Vercel status links to GitHub/Vercel authorization for the Zcorehub project, so it appears to be an integration authorization issue rather than a code/build failure.