firka: add a section for ghost grades

firka/lib/ui/components/common_bottom_sheets.dart

@@ -1245,15 +1245,6 @@ class _GradeCalculatorSheetContentState
             ),
           ),
         ),
-        if (entries.isNotEmpty) ...[
-          SizedBox(height: 16),
-          Text(
-            '${widget.data.l10n.subject_avg}: ${_weightedAverage.toStringAsFixed(2)}',
-            style: appStyle.fonts.B_14R.apply(
-              color: appStyle.colors.textPrimary,
-            ),
-          ),
-        ],
       ],
     );
   }

firka/lib/ui/phone/pages/home/home_grades_subject.dart

@@ -131,7 +131,17 @@ class _HomeGradesSubjectScreen extends FirkaState<HomeGradesSubjectScreen> {
       }).toList();
 
       var gradeWidgets = List<Widget>.empty(growable: true);
-      gradeWidgets.addAll(ghostGradeWidgets);
+      if (ghostGradeWidgets.isNotEmpty) {
+        gradeWidgets.add(
+          Text(
+            widget.data.l10n.ghost_grades,
+            style: appStyle.fonts.B_16R.apply(
+              color: appStyle.colors.textPrimary,
+            ),
+          ),
+        );
+        gradeWidgets.addAll(ghostGradeWidgets);
+      }
 
       for (var group in groups.entries) {
         gradeWidgets.add(SizedBox(height: 8));