fix(sharing): omit groups from share placeholder when group sharing disabled

[whoalin1]

Summary

When an admin disables group sharing (core / shareapi_allow_group_sharing), the calendar share search still said “Share with users or groups”.

Expose that config via calendar initial state, load it into the settings store (same path as other server settings), and omit “groups” from the share search placeholder / empty state when the flag is false. Group principals are also filtered out of DAV sharee results in that case.

Closes #2946

Test plan

• Admin → Sharing → disable “Allow sharing with groups”
• Open Calendar → share a calendar → placeholder is “Share with users” (no “groups”); empty state is “No users”
• Re-enable group sharing → placeholder returns to “Share with users or groups”; groups appear in search again
• PHP: tests/php/unit/Service/CalendarInitialStateServiceTest.php
• JS: tests/javascript/unit/store/settings.test.js

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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.

[GVodyanov]

Hey! Thanks for the PR. I think u can use the share manager for this
$allowGroupSharing = $this->shareManager->allowGroupSharing();

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[whoalin1]

@GVodyanov good call, switched to IManager::allowGroupSharing().

AI disclosure: this reply was drafted with help from an AI assistant.