fix: prevent FAB from obscuring last list items in lesson mode

[Copilot]

The category selector FAB (shown when 2+ grade categories exist) overlaps the bottom of the student list in lesson mode, blocking interaction with the last rows (e.g. adding a note to the last student).

Change

When the FAB is visible, the ListView bottom padding is increased by the extended FAB height (56 dp) plus its default margin (kFloatingActionButtonMargin = 16 dp), giving the list enough room to scroll clear of the button.

const kExtendedFabHeight = 56.0;
final hasFab = gradeCategories.length > 1;
final listPadding = hasFab
    ? appScreenPadding.copyWith(
        bottom: appScreenPadding.bottom +
            kFloatingActionButtonMargin +
            kExtendedFabHeight,
      )
    : appScreenPadding;