- Combine own and shared calendars into a single /web/calendar month
view, each event colored per its source calendar.
- New event creation now uses a calendar <select> (only writable
calendars offered); editing keeps the event's original calendar fixed.
- Shared calendars use an "owner~name" reference in URLs, resolved via
resolveCalRef which also enforces read/write permissions from
calendar_shares (read-only shares can view/export but not edit/delete).
- Add per-calendar ICS export (single event and export-all) and import,
splitting multi-VEVENT uploads into individual stored objects while
preserving VTIMEZONE definitions and source VERSION/PRODID.
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>