Refresh Spiny branding and social preview

[X1pheR]

Summary

• replace the website's Spiny assets with the accepted square versions
• rename the 404 mascot asset from dead-spiny.png to spiny-dead.png
• replace the Open Graph/social preview with the accepted 1200×630 social card
• remove the unused social-card.svg
• refresh favicon and manifest assets from the accepted production files
• align source markup, CSS, build requirements, tests, and design documentation with the new asset contracts

Validation

• ./scripts/build.sh — PASS
• Python unit tests — 6/6 PASS
• Playwright browser acceptance suite — PASS
• git diff --check — PASS
• no stale dead-spiny or social-card.svg references remain in maintained source