The server now reports per-calendar sync failures (e.g. expired
credentials) alongside an otherwise-successful events response, via an
"errors" array. Previously such failures were silently swallowed, so a
calendar could appear enabled while showing zero events with no
explanation.
- Decode the new "errors" field into SyncError (source/name/message),
following the existing JSONSerialization + from(json:) convention.
- CalendarStore.syncErrors captures these on successful personal
fetches (loadEvents, prefetchBackground); left untouched on a hard
fetch failure so it isn't conflated with lastError. Group overlay
fetches never populate it (the endpoint doesn't apply there).
- CalendarHostView shows a second red warning banner alongside the
existing lastError banner, reusing the same visual style.
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