[whoalin1]

Summary

Closes #9226

Thunderbird's built-in CardDAV address book cannot manage contact groups/lists. Document that limitation next to the recommended native setup and point users who need groups at the CardBook alternative already described on the same page.

Test plan

• Docs build for user_manual/groupware/sync_thunderbird.rst
• Note appears under Contacts / recommended method, before Calendars

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AI disclosure

This PR was drafted with the help of AI coding assistants (Cursor agents / LLM-based tools), including the description. I am responsible for it and happy to rework anything that doesn't fit.

All commits carry an Assisted-by: Cursor:grok-4.7 trailer.

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