feat(admin): student credential management, CSV import UI, remote classroom control center

- StudentsView: credential status in the roster table, a create-student form
  (email/login-ID/cohort), convert/reset-password/revoke-sessions controls per student, a
  bulk CSV import form. Every one-time activation/reset link is surfaced exactly once, never
  re-fetchable.
- ClassroomView: cohort picker -> schedule/list classes -> a live dashboard (participants with
  presence/Lab-status/current-work, help queue with acknowledge/resolve, current-activity
  setter, announcement poster, start/end/cancel, Prepare Labs). Polls the dashboard endpoint
  rather than a second push channel.
- AdminPortal: new Classroom nav entry. CohortsView's existing quick-add-student call updated
  for createStudent's new object-argument signature.

frontend/src/admin/AdminPortal.jsx

@@ -8,6 +8,7 @@ import AuditView from './AuditView'
 import AccountView from './AccountView'
 import LearningView from './LearningView'
 import AssessmentsView from './AssessmentsView'
+import ClassroomView from './ClassroomView'
 import styles from './Admin.module.css'
 
 // Instructor/Admin portal — clearly separate from the student experience (/admin route,
@@ -20,6 +21,7 @@ const NAV = [
   { key: 'dashboard', label: 'Dashboard' },
   { key: 'cohorts', label: 'Cohorts' },
   { key: 'students', label: 'Students' },
+  { key: 'classroom', label: 'Classroom' },
   { key: 'learning', label: 'Learning' },
   { key: 'assessments', label: 'Assessments' },
   { key: 'instructors', label: 'Instructors', roles: ['owner'] },
@@ -59,6 +61,7 @@ export default function AdminPortal() {
         {view === 'dashboard' && <AdminDashboard />}
         {view === 'cohorts' && <CohortsView role={instructor.role} />}
         {view === 'students' && <StudentsView role={instructor.role} />}
+        {view === 'classroom' && <ClassroomView />}
         {view === 'learning' && <LearningView />}
         {view === 'assessments' && <AssessmentsView role={instructor.role} />}
         {view === 'instructors' && instructor.role === 'owner' && <InstructorsView currentInstructorId={instructor.instructor_id} />}