Fix review issues in AdminUsersScreen and related files (task db)

- Replace identity-equality optimistic-create revert with index-based
  logic (placeholderIdx) to avoid relying on reference equality
- Replace insert(index) delete revert with append to avoid stale-index
  position jitter from concurrent mutations
- Remove _RoleBadge key collision; update test to use text finders
- currentUserProvider catch block returns null instead of User(id:0)
  to avoid colliding with the optimistic placeholder sentinel; fix
  broken comment
- adminUserErrorMessage 400-branch delegates to dioErrorMessage to
  eliminate duplicated JSON body-parsing logic
- Add Completer-backed optimistic placeholder visibility test
- _EmptyView: replace magic height SizedBox with LayoutBuilder+Center
- _CreateUserDialogState: inline _buildForm and _buildActions into build()

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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