fix(lessons): load grades reactively when category changes

The grade selections in lesson mode used a FutureProvider (one-shot
fetch). Switching categories created a new provider instance, but
any failure or the brief AsyncLoading flash appeared as empty grades
with no feedback, and there was no reactive update when a grade was
saved or cleared.

Replace with a StreamProvider backed by a new watchSessionSelections
stream method on GradeRepository. The stream uses Drift's .watch()
with readsFrom, so it emits fresh data whenever grade_entries_table or
students_table change. This matches the same reactive pattern used by
all other lesson providers (material, homework, absence).

The manual ref.invalidate calls after saveEntry / clearSessionSelection
are no longer needed and have been removed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: mikebarkmin

lib/features/grades/grade_repository.dart

@@ -170,6 +170,41 @@ class GradeRepository {
     };
   }
 
+  Stream<Map<int, String>> watchSessionSelections({
+    required int groupId,
+    required String sessionLabel,
+    required DateTime date,
+    required String categoryId,
+  }) {
+    final normalizedDate = DateTime(date.year, date.month, date.day);
+    return _database
+        .customSelect(
+          '''
+      SELECT g.student_id, g.value
+      FROM grade_entries_table g
+      JOIN students_table s ON s.id = g.student_id
+      WHERE s.group_id = ? AND g.session_label = ? AND g.date = ? AND g.category_id = ?
+      ''',
+          variables: [
+            Variable.withInt(groupId),
+            Variable.withString(sessionLabel),
+            Variable.withDateTime(normalizedDate),
+            Variable.withString(categoryId),
+          ],
+          readsFrom: {
+            _database.gradeEntriesTable,
+            _database.studentsTable,
+          },
+        )
+        .watch()
+        .map(
+          (rows) => {
+            for (final row in rows)
+              row.read<int>('student_id'): row.read<String>('value'),
+          },
+        );
+  }
+
   Future<void> saveEntry({
     required int studentId,
     required DateTime date,

lib/features/lessons/lesson_mode_screen.dart

@@ -625,14 +625,6 @@ class _LessonModeScreenState extends ConsumerState<LessonModeScreen> {
           categoryId: category.id,
           categoryName: category.name,
         );
-    ref.invalidate(
-      lessonGradeSelectionsProvider((
-        widget.groupId,
-        _selectedDate,
-        sessionLabel,
-        category.id,
-      )),
-    );
   }
 
   Future<void> _clearGrade({
@@ -648,14 +640,6 @@ class _LessonModeScreenState extends ConsumerState<LessonModeScreen> {
           sessionLabel: sessionLabel,
           categoryId: category.id,
         );
-    ref.invalidate(
-      lessonGradeSelectionsProvider((
-        widget.groupId,
-        _selectedDate,
-        sessionLabel,
-        category.id,
-      )),
-    );
   }
 
   Future<void> _pickGrade({

lib/features/lessons/lesson_support.dart

@@ -70,11 +70,11 @@ final lessonEntryDatesProvider = StreamProvider.autoDispose
           ref.watch(lessonRepositoryProvider).watchGroupEntryDates(groupId),
     );
 
-final lessonGradeSelectionsProvider = FutureProvider.autoDispose
+final lessonGradeSelectionsProvider = StreamProvider.autoDispose
     .family<Map<int, String>, (int, DateTime, String, String)>(
       (ref, args) => ref
           .watch(gradeRepositoryProvider)
-          .getSessionSelections(
+          .watchSessionSelections(
             groupId: args.$1,
             date: normalizeLessonDate(args.$2),
             sessionLabel: args.$3,