feat(student): add learning tabs and Lab hint/assessment UX

StudentDashboard is now tabbed: My Learning (the original Phase 2A/2B flow, extracted
unchanged into MyLearningTab and extended with an overall percent-complete bar), Practice,
Assessments (available/active-with-live-countdown/history), Capstone (locked checklist ->
unlocked -> start/resume -> completed), Achievements (a flat dated list, not a leaderboard).

Inside the Lab UI: ProgressiveHints replaces the static hint array render when the Gateway has
stripped hints (detected via scenario.hints being undefined but hint_count present - standalone
dev mode never sees this, so it keeps the original behavior); a new top-level AssessmentBanner
shows a live countdown and completion count whenever a Platform assessment is running,
rendered alongside (not replacing) the pre-existing Lab-local exam-mode UI, which is untouched.

frontend/src/App.jsx

@@ -8,6 +8,7 @@ import ExamTimer from './components/ExamTimer'
 import ExamReport from './components/ExamReport'
 import ExamStartModal from './components/ExamStartModal'
 import ExamHistory from './components/ExamHistory'
+import AssessmentBanner from './components/AssessmentBanner'
 import styles from './App.module.css'
 
 const MIN_SIDEBAR_W = 180
@@ -303,6 +304,7 @@ export default function App() {
   return (
     <div className={styles.app}>
       <Header clusterReady={clusterReady} onShowHistory={() => setShowHistory(true)} />
+      <AssessmentBanner />
 
       {/* Bundle navigation bar */}
       <BundleNav