Distinguish an empty calendar from a failed calendar fetch

**Problem**

`fetchEvents` returns an empty list both when the request succeeds with no events and when it fails with the `-1` or `-2` sentinel. The calendar view has neither an error state nor an empty state.

A student with no connectivity sees an ordinary empty calendar and concludes there are no events.

**Where**

- `lib/services/generic/calendarService.dart:12-23`
- `lib/views/calendarView.dart:114-138`

**Expected**

A network failure shows the retry widget, as the other tabs already do. No events says so explicitly.

**Fix sketch**

- Return `List?` and render `ErrorWidgetWithRefreshCallback` on null.
- Add an explicit empty state.

Start in lib/services/generic/calendarService.dart:12-23 to trace how fetchEvents represents success and the -1/-2 failures, then inspect lib/views/calendarView.dart:114-138 and the retry handling used by other tabs. Done means network failures show ErrorWidgetWithRefreshCallback and successful responses with no events show an explicit empty state.

Written by the indexing model from the issue text.