fix(entries): reject create/update of intervals in the future

- Add ErrFutureDay sentinel error
- CreateInterval: rejects if startDayKey > todayKey (400)
- Update: rejects if new start_time moves entry to a future day (400)
- Handler maps ErrFutureDay → 400 Bad Request for both endpoints
- Add TestCreateIntervalRejectsFutureDay
- Add TestUpdateRejectsMoveToFutureDay
- UI already gates this via dayCapabilities (canAddInterval=false,
  canEditEntries=false for future days), but server now enforces it too

internal/service/entry_service_test.go

@@ -231,3 +231,55 @@ func TestUpdateRejectsClosedDay(t *testing.T) {
 		t.Fatalf("expected ErrDayAlreadyClosed, got %v", err)
 	}
 }
+
+func TestCreateIntervalRejectsFutureDay(t *testing.T) {
+	ctx := context.Background()
+	svc := newTestServices(t)
+
+	// Build a start_time that is tomorrow.
+	tomorrow := time.Now().UTC().AddDate(0, 0, 1)
+	startMs := time.Date(tomorrow.Year(), tomorrow.Month(), tomorrow.Day(), 9, 0, 0, 0, time.UTC).UnixMilli()
+	endMs := startMs + 3_600_000 // +1h
+
+	_, err := svc.CreateInterval(ctx, service.CreateIntervalInput{
+		StartTime: startMs,
+		EndTime:   endMs,
+		Note:      "future",
+	})
+	if err == nil {
+		t.Fatal("expected error creating interval in the future")
+	}
+	if !errors.Is(err, service.ErrFutureDay) {
+		t.Fatalf("expected ErrFutureDay, got %v", err)
+	}
+}
+
+func TestUpdateRejectsMoveToFutureDay(t *testing.T) {
+	ctx := context.Background()
+	svc := newTestServices(t)
+
+	// Create a valid interval for today.
+	now := time.Now().UTC()
+	startMs := time.Date(now.Year(), now.Month(), now.Day(), 8, 0, 0, 0, time.UTC).UnixMilli()
+	endMs := startMs + 3_600_000
+	entry, err := svc.CreateInterval(ctx, service.CreateIntervalInput{
+		StartTime: startMs,
+		EndTime:   endMs,
+	})
+	if err != nil {
+		t.Fatalf("CreateInterval: %v", err)
+	}
+
+	// Try to move start_time to tomorrow.
+	tomorrow := now.AddDate(0, 0, 1)
+	futureStart := time.Date(tomorrow.Year(), tomorrow.Month(), tomorrow.Day(), 8, 0, 0, 0, time.UTC).UnixMilli()
+	_, err = svc.Update(ctx, entry.ID, service.UpdateEntryInput{
+		StartTime: &futureStart,
+	})
+	if err == nil {
+		t.Fatal("expected error moving entry to future day")
+	}
+	if !errors.Is(err, service.ErrFutureDay) {
+		t.Fatalf("expected ErrFutureDay, got %v", err)
+	}
+}