Fix mobile sponsor logo sizing and spacing

[devin-ai-integration]

Summary

A previous attempt to make Gold/Blue sponsor logos smaller and more condensed on mobile had no visible effect. Two root causes:

1. Whitespace wasn't controlled by gap. The mobile grids used grid-cols-N + justify-items-center, which creates equal-width columns (each 1/N of the container) with the logo centered inside. The visible space between logos came from the wide columns, so shrinking gap-x did almost nothing.
2. Sizing didn't take. Logos render at their natural size when it's below the max-w/max-h cap, so reducing max-h-12 → max-h-10 was barely noticeable. Separately, Diamond used max-h-18, which isn't a real Tailwind class (the default scale skips 18) so it never compiled.

Fixes (all in content/_index.md, mobile only — desktop reverts via sm: and is unchanged):

• Columns are now content-sized so the gap is the actual spacing:
  grid-cols-[repeat(2,auto)] / grid-cols-[repeat(3,auto)] + justify-center.
• Logos use a fixed box + object-contain so they actually scale to the requested size (up for Diamond, down for Gold/Blue), e.g.
  • Diamond: w-32 h-16 object-contain (larger)
  • Gold: w-28 h-10 object-contain (smaller) + tighter gap-x-4
  • Blue: w-20 h-10 object-contain (smaller) + tighter gap-x-3
• Gold's lone third logo (ContestDojo) is centered on its second row via [&>a:last-child]:col-span-2 [&>a:last-child]:justify-self-center.

Verified at ~390px (mobile) and full width (desktop).

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

[devin-ai-integration]

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