Add theme social links

[minimaluminium]

Summary

• add publication social links to theme footers and related social surfaces
• add/update platform icons, including X replacement for Twitter icons
• bump theme shared asset pins to 2.7.1 and include rebuilt CSS assets/maps
• bump GScan to 6.2.0 and theme Ghost engines to >=6.38.0 for the new social_accounts helper

Testing

• yarn install --ignore-scripts
• yarn build
• yarn test:ci --theme alto
• gscan --fatal for all 16 themes
• git diff --cached --check

Stacked on #513 and targeted at codex/shared-social-links so this PR only shows theme consumer changes. Locale/i18n output is intentionally excluded.

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