feat(student): live student experience — real login, classroom, profile

- LoginScreen (credentialed, now the default entry point) with links to legacy pilot entry
  and forgot-password; SetPasswordScreen (shared activate/reset UI, driven by
  ?activate=/?reset= query params — no router library, matching this app's existing
  state-machine-per-shell convention); ForgotPasswordScreen (anti-enumeration, dev-only
  reset-token surface).
- ClassroomTab: LIVE NOW banner, join, heartbeat only while joined and the tab is mounted,
  guidance-only current-activity link, own Lab status/launch (reuses the My Learning tab's
  exact API), announcements, Need Help form. No chat, no video, no shell access.
- ProfileTab: identity, cohort, progress, achievements, active-session count,
  change-password (gated on has_credentials).
- PlatformShell rewritten as an explicit view state machine; StudentDashboard gained the new
  tabs; api.js/Platform.module.css extended to match.

frontend/src/platform/StudentDashboard.jsx

@@ -6,13 +6,17 @@ import PracticeTab from './PracticeTab'
 import AssessmentsTab from './AssessmentsTab'
 import CapstoneTab from './CapstoneTab'
 import AchievementsTab from './AchievementsTab'
+import ClassroomTab from './ClassroomTab'
+import ProfileTab from './ProfileTab'
 
 const TABS = [
   { key: 'learning', label: 'My Learning' },
+  { key: 'classroom', label: 'Classroom' },
   { key: 'practice', label: 'Practice' },
   { key: 'assessments', label: 'Assessments' },
   { key: 'capstone', label: 'Capstone' },
   { key: 'achievements', label: 'Achievements' },
+  { key: 'profile', label: 'Profile' },
 ]
 
 // Student home — tabbed as of Phase 3B (docs/PHASE3B_LEARNING_AND_ASSESSMENT.md §10): My
@@ -41,10 +45,12 @@ export default function StudentDashboard({ student, onLogout }) {
       </nav>
 
       {tab === 'learning' && <MyLearningTab />}
+      {tab === 'classroom' && <ClassroomTab />}
       {tab === 'practice' && <PracticeTab />}
       {tab === 'assessments' && <AssessmentsTab />}
       {tab === 'capstone' && <CapstoneTab />}
       {tab === 'achievements' && <AchievementsTab />}
+      {tab === 'profile' && <ProfileTab />}
     </div>
   )
 }