Turn ContributionCard into a container to allow more fluid sizing

[MathijsR94]

Summary

• Transform ContributionCard into a CSS container query component so the header layout adapts to the card's own width instead of the viewport
• At narrow card widths (<320px), the header stacks vertically
• At medium widths (320–399px), the "contributions" label is hidden, showing only the number to save space

Test plan

• Resize the browser with 1 user loaded (full-width card) — header stays horizontal
• Load 3–4 users so cards become narrow — verify header stacks/truncates gracefully

[brdv]

Smoooooth