- Parse the server's new read_only flag on events.
- Filter sheet: a calendar shared with me is listed under the owner's name
(sharedBy) with a lock icon when I only have read access.
- loadWritableCalendars: exclude read-only shared calendars so they no longer
appear in the event editor (a save would 403).
The group-title prefix removal is server-side and needs no iOS change.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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