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/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) {