Gate Typesense search on API key

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Summary

• remove the hard-coded placeholder fallback for the Typesense API key
• enable the Typesense theme, config, and search navbar item only when TYPESENSE_API_KEY is set
• keep production/search-enabled builds working with the existing Typesense server settings

Verification

• rg -n "placeholder|TYPESENSE_API_KEY|typesenseApiKey|type: 'search'|docusaurus-theme-search-typesense" docusaurus.config.ts -S\n- git diff --check\n- env -u TYPESENSE_API_KEY npm run build\n- TYPESENSE_API_KEY=dummy-search-key npm run build\n\nBoth builds succeed; remaining broken link/anchor warnings are the known main warnings addressed in Fix docs link checks and enforce quality gates #115/Fix Cockpit process instance task wording #118. Ignored generated build/cache artifacts were removed afterward to avoid local ENOSPC issues.\n\nReferences checked:\n- Local docusaurus-theme-search-typesense README and theme validation source under node_modules/docusaurus-theme-search-typesense\n- https://typesense.org/docs/guide/docsearch.html\n