Restyle mailing-list signup and make sponsor grids responsive

[devin-ai-integration]

Summary

Homepage tweaks to the mailing-list signup and the sponsor section, all in content/_index.md.

Mailing-list signup form:

• The "Subscribe" submit input previously relied on Mailchimp's class="button" and rendered unstyled. It now uses the site's teal button styling (matching "Contact Us"), via Tailwind utilities with !important to override Mailchimp's CSS.
• The email <input> gets a clearer outline (!border !border-gray-400 !rounded) since the previous white box was nearly invisible on the light background.
• Email box + button are wrapped in flex flex-col sm:flex-row so the button sits to the right of the email box on sm+ and stacks underneath on mobile.

Sponsor tiers — per-row splits on small screens: each tier's div_class switches to a CSS grid on mobile and reverts to the existing centered flex row on sm+:

• Diamond (4): grid grid-cols-2 → 12 / 34
• Gold (3): grid grid-cols-2 [&>a:first-child]:col-span-2 → 1 / 23
• Blue (6): grid grid-cols-3 → 123 / 456

Verified locally at mobile (~390px) and desktop widths; desktop layout is unchanged.

Link to Devin session: https://app.devin.ai/sessions/2c62d21ae9ab454ca4fe0b9e0d559825

[devin-ai-integration]

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