fix: add validateExam to loadSample; use native button for dropzone accessibility

frontend/src/components/Landing.tsx

@@ -41,6 +41,8 @@ export default function Landing({ onLoadExam, history, onResume, onClearHistory,
     try {
       const r = await fetch(path)
       const data = await r.json() as unknown
+      const msg = validateExam(data)
+      if (msg) throw new Error(msg)
       onLoadExam(parseExam(data))
     } catch (e) {
       setErr(`Could not load sample: ${(e as Error).message}`)
@@ -59,7 +61,8 @@ export default function Landing({ onLoadExam, history, onResume, onClearHistory,
 
       <div className="landing-grid">
         <div>
-          <div
+          <button
+            type="button"
             className={`dropzone${dragOver ? ' drag-over' : ''}`}
             onClick={() => fileRef.current?.click()}
             onDragOver={(e) => { e.preventDefault(); setDragOver(true) }}
@@ -80,7 +83,7 @@ export default function Landing({ onLoadExam, history, onResume, onClearHistory,
               style={{ display: 'none' }}
               onChange={(e) => { const f = e.target.files?.[0]; if (f) void handleFile(f) }}
             />
-          </div>
+          </button>
 
           {err && (
             <div style={{ marginTop: 12, padding: '10px 14px', border: '1px solid var(--wrong)', borderRadius: 8, background: 'var(--wrong-weak)', color: 'var(--wrong)', fontSize: 13 }}>