Reset stale reading challenges after navigation

public/reader.js

@@ -43,7 +43,10 @@
             if (result.complete) { message.textContent = 'Unlocked. Opening your chapter…'; location.reload(); return; }
             message.textContent = 'Correct.';
             showQuestion(result);
-          } catch (error) { message.textContent = error.message; }
+          } catch (error) {
+            message.textContent = error.message;
+            if (/challenge expired/i.test(error.message)) { challenge = null; question.textContent = ''; start.hidden = false; }
+          }
         });
         question.append(form); input.focus();
       };
@@ -55,6 +58,7 @@
       window.addEventListener('pagehide', () => {
         if (challenge) navigator.sendBeacon('api/reading-quiz/abandon', new Blob([JSON.stringify({ challenge })], { type: 'application/json' }));
       });
+      window.addEventListener('pageshow', event => { if (event.persisted) location.reload(); });
       setInterval(async () => {
         try { if ((await api('api/reading-access?' + qsa({ m: P.get('m') }))).allowed) location.reload(); } catch { /* keep the gate visible */ }
       }, 15000);
@@ -67,9 +71,11 @@
   catch (e) { canvas.textContent = ''; canvas.append(h('p', { class: 'loading' }, e.message + ' · ', h('a', { href: './' }, 'Library'))); return; }
 
   // An already-open chapter also closes when a window ends or an unlock expires.
-  setInterval(async () => {
+  const checkReadingAccess = async () => {
     try { if (!(await api('api/reading-access?' + qsa({ m: P.get('m') }))).allowed) location.reload(); } catch { /* transient check failure */ }
-  }, 15000);
+  };
+  setInterval(checkReadingAccess, 15000);
+  document.addEventListener('visibilitychange', () => { if (!document.hidden) checkReadingAccess(); });
 
   const S = me.settings, rtl = D.direction === 'rtl', ttb = D.direction === 'ttb';
   const GAP = ttb || S.flowPageGap === false ? 0 : 8, END_H = 280;