Fix CBT session isolation and backend seed mock data exam pattern

src/app/cbt/[examId]/page.tsx

@@ -151,11 +151,11 @@ export default function CBTExamPage({ params }: { params: Promise<{ examId: stri
           };
         }).filter(Boolean) as any[];
 
-        initExam(examId, fetchedQuestions, testData.total_time * 60);
+        initExam(examId, fetchedQuestions, testData.total_time * 60, userId);
         setMounted(true);
       } catch (err) {
         console.error("Failed to load exam from network, falling back to dummy data:", err);
-        initExam(examId, DUMMY_QUESTIONS, 10800);
+        initExam(examId, DUMMY_QUESTIONS, 10800, userId);
         setMounted(true);
       }
     }

src/store/cbtStore.ts

@@ -46,6 +46,7 @@ export interface ExamAttemptPayload {
 }
 
 interface CBTState {
+  userId: string | null
   examId: string | null
   attemptId: string | null
   status: 'NOT_STARTED' | 'IN_PROGRESS' | 'SUBMITTED' | 'TIMED_OUT'
@@ -56,7 +57,7 @@ interface CBTState {
   currentQuestionIndex: number
   
   // Actions
-  initExam: (examId: string, questions: any[], duration: number) => void
+  initExam: (examId: string, questions: any[], duration: number, userId: string) => void
   startExam: () => void
   selectAnswer: (questionId: string, answer: string) => void
   clearResponse: (questionId: string) => void
@@ -72,6 +73,7 @@ interface CBTState {
 export const useCBTStore = create<CBTState>()(
   persist(
     (set, get) => ({
+      userId: null,
       examId: null,
       attemptId: null,
       status: 'NOT_STARTED',
@@ -81,12 +83,13 @@ export const useCBTStore = create<CBTState>()(
       answers: {},
       currentQuestionIndex: 0,
 
-      initExam: (examId, questions, duration) => {
+      initExam: (examId, questions, duration, userId) => {
         const currentExamId = get().examId;
+        const currentUserId = get().userId;
         const currentQuestions = get().questions;
         
-        // Force reset if it's a new exam or the number of questions has changed
-        const needsReset = currentExamId !== examId || currentQuestions.length !== questions.length;
+        // Force reset if it's a new exam, new user, or the number of questions has changed
+        const needsReset = currentExamId !== examId || currentUserId !== userId || currentQuestions.length !== questions.length;
 
         if (needsReset) {
           const initialAnswers: Record<string, AnswerState> = {};
@@ -100,6 +103,7 @@ export const useCBTStore = create<CBTState>()(
           })
 
           set({
+            userId,
             examId,
             attemptId: null,
             status: 'NOT_STARTED',
@@ -271,6 +275,7 @@ export const useCBTStore = create<CBTState>()(
 
       resetExamState: () => {
         set({
+          userId: null,
           examId: null,
           attemptId: null,
           status: 'NOT_STARTED',
@@ -286,6 +291,7 @@ export const useCBTStore = create<CBTState>()(
       name: 'cbt-storage', // unique name
       storage: createJSONStorage(() => idbStorage), // use custom IndexedDB storage
       partialize: (state) => ({
+        userId: state.userId,
         examId: state.examId,
         status: state.status,
         startTime: state.startTime,