feat(frontend): lesson stats panel + sublesson list + recent sessions list

packages/frontend/src/components/LessonStatsPanel.tsx

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+import type { LessonStats } from '../api/stats.js';
+
+function relativeTime(unixSec: number | null): string {
+  if (!unixSec) return 'nooit';
+  const diff = Math.floor(Date.now() / 1000) - unixSec;
+  if (diff < 3600) return `${Math.max(1, Math.floor(diff / 60))}m geleden`;
+  if (diff < 86400) return `${Math.floor(diff / 3600)}u geleden`;
+  return `${Math.floor(diff / 86400)}d geleden`;
+}
+
+export function LessonStatsPanel({ stats, lastSessionAt }: { stats: LessonStats; lastSessionAt?: number | null }) {
+  const masteredFrac = stats.totalCards === 0 ? 0 : stats.mastered / stats.totalCards;
+  return (
+    <div className="grid grid-cols-2 gap-3 sm:grid-cols-4">
+      <Card label="Kaarten" value={String(stats.totalCards)} />
+      <Card label="Beheerst" value={`${stats.mastered}/${stats.totalCards}`} sub={`${Math.round(masteredFrac * 100)}%`} />
+      <Card label="Score" value={`${Math.round(stats.score * 100)}%`} />
+      <Card label="Laatst geoefend" value={relativeTime(lastSessionAt ?? null)} />
+    </div>
+  );
+}
+
+function Card({ label, value, sub }: { label: string; value: string; sub?: string }) {
+  return (
+    <div className="surface p-4">
+      <div className="text-[10px] font-semibold uppercase tracking-wider text-slate-500">{label}</div>
+      <div className="mt-1 font-display text-2xl font-bold">{value}</div>
+      {sub && <div className="text-xs text-slate-500">{sub}</div>}
+    </div>
+  );
+}

packages/frontend/src/components/RecentSessionsList.tsx

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+function relativeTime(unixSec: number): string {
+  const diff = Math.floor(Date.now() / 1000) - unixSec;
+  if (diff < 3600) return `${Math.max(1, Math.floor(diff / 60))}m geleden`;
+  if (diff < 86400) return `${Math.floor(diff / 3600)}u geleden`;
+  return `${Math.floor(diff / 86400)}d geleden`;
+}
+
+function fmtDuration(s: number): string {
+  if (s < 60) return `${s}s`;
+  return `${Math.floor(s / 60)}m ${s % 60}s`;
+}
+
+export interface RecentSessionRow {
+  id: number;
+  startedAt: number;
+  durationSeconds: number | null;
+  cardsShown: number;
+  cardsCorrect: number;
+}
+
+export function RecentSessionsList({ rows }: { rows: RecentSessionRow[] }) {
+  if (rows.length === 0) {
+    return <p className="text-sm text-slate-500">Nog geen sessies op deze les.</p>;
+  }
+  return (
+    <ul className="surface divide-y divide-brand-100/60 dark:divide-slate-800">
+      {rows.map((s) => {
+        const pct = s.cardsShown > 0 ? Math.round((s.cardsCorrect / s.cardsShown) * 100) : 0;
+        return (
+          <li key={s.id} className="flex items-center justify-between p-3 text-sm">
+            <span className="text-slate-600 dark:text-slate-300">{relativeTime(s.startedAt)}</span>
+            <span className="flex items-center gap-3 text-xs">
+              <span className="rounded-full bg-brand-100 px-2 py-0.5 font-semibold text-brand-700 dark:bg-brand-900/30 dark:text-brand-200">{pct}%</span>
+              <span className="text-slate-500">{s.cardsCorrect}/{s.cardsShown} · {fmtDuration(s.durationSeconds ?? 0)}</span>
+            </span>
+          </li>
+        );
+      })}
+    </ul>
+  );
+}

packages/frontend/src/components/SublessonList.tsx

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+import { Link } from 'react-router-dom';
+import type { LessonTreeNode } from '@flashcard/shared';
+
+export function SublessonList({ items }: { items: LessonTreeNode[] }) {
+  if (items.length === 0) return null;
+  return (
+    <div>
+      <h2 className="mb-3 font-display text-xl font-bold">Sublessen</h2>
+      <ul className="grid gap-2 sm:grid-cols-2">
+        {items.map((c) => (
+          <li key={c.id} className="surface flex items-center justify-between p-3">
+            <Link to={`/lessons/${c.id}`} className="flex-1 truncate font-medium">{c.name}</Link>
+            <span className="rounded-full bg-brand-100 px-2 py-0.5 text-xs font-semibold text-brand-700 dark:bg-brand-900/30 dark:text-brand-200">
+              {c.cardCount}
+            </span>
+          </li>
+        ))}
+      </ul>
+    </div>
+  );
+}