feat: complete exam lifecycle — AI generation, submission, student session, and results

- Backend: AI generation fallbacks when OpenAI not configured, full exam
  submission saving all params (difficulty, rubric, entity, grading system,
  approval workflow) and creating linked question records per section
- Backend: new exam session controller with get_session, autosave, submit,
  status, and results endpoints; student attempt/answer/score models
- Backend: new controllers for entities, approval workflows, exam schedules
- Frontend: exam session split-layout with passage panel, question types
  (MCQ, T/F/NG, gap-fill, writing, speaking), timer, and review dialog
- Frontend: results page with percentage score, per-answer breakdown table
- Frontend: generation page dynamic dropdowns, full payload submission
- Frontend: updated types for ExamSessionSection, ExamQuestion options

Made-with: Cursor

src/components/StudentLayout.tsx

@@ -1,4 +1,5 @@
 import RoleLayout, { NavGroup } from "./RoleLayout";
+import ExamPopup from "./student/ExamPopup";
 import {
   LayoutDashboard, BookOpen, ClipboardList, BarChart3,
   CalendarCheck, Calendar, User, Target, GraduationCap, ListChecks,
@@ -47,5 +48,10 @@ const navGroups: NavGroup[] = [
 ];
 
 export default function StudentLayout() {
-  return <RoleLayout navGroups={navGroups} role="student" />;
+  return (
+    <>
+      <RoleLayout navGroups={navGroups} role="student" />
+      <ExamPopup />
+    </>
+  );
 }