A calendar holding only tasks (VTODO, e.g. from tasks.org) appeared in the
event calendar UI as an empty calendar. Stalwart's Calendar/get exposes no
supported-component set, so tasks-only-ness is derived from the calendar's
objects (mirroring the existing event/task split): a calendar is tasks-only
iff it holds >=1 object and every object is a task.
- getTasksOnlyCalendarIds scans an account's objects (CalendarEvent/query +
minimal /get), early-exits once every calendar has an event, only marks
after a full scan, and fails open on the cap or any error.
- getAllCalendars/getCalendars annotate calendars with isTasksOnly.
- Such calendars stay in the store for the tasks view (task picker, task-list
grouping) but are filtered from the event sidebar and default selection.
Verified against Stalwart v0.16.16: the tasks-only calendar is flagged, the
default (with an event) is not.
Closes#761.
