Build Chapter Assignment Editor end-affiliation flow

Use showModalBottomSheet with isDismissible: true so swipe-to-dismiss works, and handle the null return value from the future (user dismissed without confirming) identically to Cancel. Model the bottom sheet as a self-contained StatefulWidget so local form state (selectedDate, noteText) is encapsulated — do not push this transient state into the cubit. Pre-fill the date picker with DateTime.now().toUtc() and clamp selectable range using firstDate: affiliation.startDate, lastDate: DateTime.now(). Use the design system AppDatePicker widget for consistent styling.

After Confirm, call cubit.trackPendingRemove before Navigator.pop so the cubit state update triggers before the sheet closes (prevents brief flicker of un-updated row). For the note field, use a TextEditingController with a maxLength enforced in the AppTextField widget. Ensure the PendingRemovalBadge on pending rows clearly communicates the pending state to screen readers via a Semantics label such as 'Affiliation end pending on {endDate}'.

Write widget tests using flutter_test: (1) mount the end-affiliation list section with a mock cubit emitting a loaded state containing 3 active affiliations and assert 3 rows with End Affiliation buttons are rendered, (2) tap End Affiliation on one row and assert the bottom sheet opens, (3) select a date in the date picker and enter a note, tap Confirm, assert trackPendingRemove is called with correct affiliationId, endDate, and note, (4) open the bottom sheet and tap Cancel, assert no cubit call was made, (5) simulate a row where pendingRemoval is already set and assert the button is disabled/replaced. Write bloc_test unit tests for trackPendingRemove cubit method. Write a golden test for the bottom sheet in its default state. Test that the date picker rejects dates before the affiliation start date.

Target 90% branch coverage.