feat: explain verified membership on landing page

[newmattock]

Summary

• Add a responsive Verified Membership section to the landing page.
• Explain the membership value with benefits already present in the current app experience: verified-only opportunities, a verified profile, more submissions, and lower bounty fees.
• Link visitors directly to the existing subscription page with a sign-in-required note.

Why

The landing page currently explains how to find work and how to find talent, but it does not explain the membership path exposed by the product. This section gives visitors a clear, factual route to learn more without inventing pricing or unsupported plan details.

Related issue: #145

Validation

• git diff --check passes.
• Copy was checked against the current public subscription UI bundle.
• A production build could not be completed in this environment. npm ci is currently rejected because the committed lockfile is out of sync with package.json; the dependency-install fallback then exhausted the host volume. No lockfile or generated build files were changed.

Scope notes

This PR is limited to the landing-page explanation and CTA. It does not alter subscription logic, authentication, billing, or payout behavior.

[vercel]

@newmattock is attempting to deploy a commit to the gibwork Team on Vercel.

A member of the Team first needs to authorize it.