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Add configurable custom menu items that appear in sidebar, each rendering
an iframe-embedded external page. Includes shared URL builder with
src_host/src_url tracking, CSP frame-src multi-origin deduplication,
admin settings UI, and i18n support.

chore: bump version to 0.1.87.19
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>