docs: update to geistdocs 1.8.0 and drop Flags from OSS dropdown (#407)

* docs: update to geistdocs 1.8.0 and drop Flags from OSS dropdown

Bump @vercel/geistdocs to 1.8.0 (eve logo, Streamdown dropped, AI Elements
last) and override navbarOssProducts to exclude Flags SDK on its own site.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* docs: link eve to /docs in OSS dropdown

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>