Peer Mentor Multi-Select screen scaffold and checkbox list rendering

Reuse the PeerMentorListTile widget created in task-005 and extend it to accept an onChanged callback and a isSelected boolean for the checkbox variant — this avoids duplicating row rendering logic. Manage the selection Set in a dedicated Riverpod StateNotifier (MultiSelectorNotifier) with add/remove/toggle methods. To prevent the entire list from rebuilding on each checkbox toggle, use Consumer at the row level watching only the specific mentor's selected state via provider.select((state) => state.selectedMentorIds.contains(mentorId)); this ensures only the toggled row rebuilds. The selection counter in the AppBar should watch only the selectedMentorIds.length via a select(), again to avoid full-screen rebuilds.

The Proxy Contact List Provider should be declared as a global/family provider so both this screen and the single selector share the same cached data — do not create a second provider with the same Supabase query.

Widget tests covering: (1) loading state — mock provider returns AsyncLoading, assert loading indicator present; (2) populated state — mock provider returns 3 mentors, assert all rows rendered with unchecked checkboxes; (3) empty state — assert empty state message; (4) error state — assert error message and retry; (5) checkbox toggle — tap row 1, assert selectedMentorIds contains row 1's ID; (6) multiple selection — tap rows 1 and 3, assert both IDs are in selectedMentorIds and rows 2 and 4 are unchecked; (7) deselect — tap a checked row, assert ID is removed from set; (8) selection count display — assert count label shows '2 selected' after two taps. Use Riverpod ProviderScope overrides with stubbed providers. Verify shared provider: assert same provider family key is used as in single selector (no duplicate).