fix(exercise): show correct submitted state and real questions for students

exercise.svelte was auto-adding a QUESTION_TEMPLATE to the store whenever
questions were empty, which raced with hydrateExercisePageData.
applyMySubmission fired with the blank template in place, set
isFinished=true, and the guard prevented re-application once real
questions loaded.

Two fixes: guard the auto-add effect with isOrgStudent (students never
need to add questions), and gate applyMySubmission on isExerciseLoaded
so it always waits for hydrateExercisePageData to populate real questions
first.

Also fix off-by-1 in applyMySubmission (questions.length → questions.length + 1)
so currentQuestion is undefined at end-of-exercise, satisfying the
isFinished derived. Clear questionnaire.questions in onDestroy to prevent
stale questions flashing on the next exercise.

Fix lesson delete not updating sidebar/content list by filtering the
removed lesson from sections on success.

apps/dashboard/src/lib/features/course/api/lesson.svelte.ts

@@ -204,6 +204,10 @@ export class LessonApi extends BaseApiWithErrors {
       logContext: 'deleting lesson',
       onSuccess: (response) => {
         if (response.data) {
+          this.sections = this.sections.map((section) => ({
+            ...section,
+            lessons: section.lessons.filter((l) => l.id !== lessonId)
+          }));
           snackbar.success('snackbar.lessons.lesson_deleted');
           this.success = true;
           this.errors = {};

apps/dashboard/src/lib/features/course/components/exercise/view-mode.svelte

@@ -649,6 +649,7 @@
 
   $effect(() => {
     if (!exerciseId || isFetchingExercise || skipHydrateFromSubmissions) return;
+    if (!isExerciseLoaded) return;
     if (!submissionList.length || $questionnaireMetaData.isFinished) return;
     let idx = selectedTryIndex;
     if (idx < 0 || idx >= submissionList.length) {
@@ -700,7 +701,7 @@
       ...m,
       answers,
       totalPossibleGrade,
-      currentQuestionIndex: $questionnaire.questions.length,
+      currentQuestionIndex: $questionnaire.questions.length + 1,
       currentSectionIndex: activeSections.length > 0 ? activeSections.length - 1 : 0,
       sectionPhase: 'questions',
       isFinished: true,

apps/dashboard/src/lib/features/course/pages/exercise.svelte

@@ -269,6 +269,7 @@
 
   onDestroy(() => {
     reset();
+    questionnaire.update((q) => ({ ...q, questions: [] }));
   });
 
   onMount(() => {
@@ -309,6 +310,8 @@
   });
 
   $effect(() => {
+    if ($isOrgStudent) return;
+
     const addNewQ = $questionnaire?.questions?.length < 1;
 
     if (addNewQ) {