feat: add seating plans for learning groups

LehrerApp.Desktop.Tests/Fakes.cs

@@ -60,6 +60,20 @@ public class FakeMemberships(List<GroupMembership> all) : IGroupMembershipReposi
     public void Delete(Guid id) => all.RemoveAll(m => m.Id == id);
 }
 
+public class FakeSeatingPlans(List<SeatingPlan>? initial = null) : ISeatingPlanRepository
+{
+    private readonly List<SeatingPlan> _all = initial ?? [];
+    public SeatingPlan? GetById(Guid id) => _all.FirstOrDefault(p => p.Id == id);
+    public List<SeatingPlan> GetByGroup(Guid groupId) =>
+        _all.Where(p => p.GroupId == groupId).OrderBy(p => p.Name).ToList();
+    public void Save(SeatingPlan plan)
+    {
+        _all.RemoveAll(p => p.Id == plan.Id);
+        _all.Add(plan);
+    }
+    public void Delete(Guid id) => _all.RemoveAll(p => p.Id == id);
+}
+
 public class FakeSessions(List<ParticipationSession> all) : IParticipationSessionRepository
 {
     public List<ParticipationSession> GetByGroup(Guid groupId) => all.Where(s => s.GroupId == groupId).ToList();