Fix TTS audio stalling and add back/previous-word navigation in spelling

- Add timeout fallback in ttsDrain so the queue never permanently stalls
  when the browser fails to fire onend (common Chrome/Safari bug after
  extended use)
- Call speechSynthesis.resume() before speak() to un-stick suspended
  synthesis on mobile
- Rename "← Stop" / "← Cancel" to "← Back" on practice and test views
- Add "← Previous word" button in spelling test so kids can go back one
  word (e.g. when audio broke) — pops the saved answer and replays

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/app.js

@@ -122,22 +122,30 @@ function ttsCancel() {
 async function ttsDrain() {
   if (!ttsQueue.length) { ttsCurrentAudio = null; return; }
   const text = ttsQueue.shift();
+  // Fallback timeout in case browser never fires onend (common speechSynthesis bug)
+  const timeoutMs = Math.max(4000, text.length * 80);
 
   if (piperAvailable) {
     const audio = new Audio(`/api/tts?text=${encodeURIComponent(text)}`);
     ttsCurrentAudio = audio;
     await new Promise((resolve) => {
-      audio.onended = resolve;
+      let t = setTimeout(resolve, timeoutMs);
-      audio.onerror = resolve;
+      const done = () => { clearTimeout(t); resolve(); };
-      audio.play().catch(resolve);
+      audio.onended = done;
+      audio.onerror = done;
+      audio.play().catch(done);
     });
   } else {
     const u = new SpeechSynthesisUtterance(text);
     u.rate = 0.85; u.lang = 'en-US';
     speechSynthesis.cancel();
     await new Promise((resolve) => {
-      u.onend = resolve;
+      let t = setTimeout(resolve, timeoutMs);
-      u.onerror = resolve;
+      const done = () => { clearTimeout(t); resolve(); };
+      u.onend = done;
+      u.onerror = done;
+      // Un-stick suspended synthesis (common on mobile after a few minutes)
+      if (speechSynthesis.paused) speechSynthesis.resume();
       // some browsers need a short delay after cancel() before speak() works
       setTimeout(() => speechSynthesis.speak(u), 50);
     });
@@ -2386,6 +2394,7 @@ function presentTestWord() {
   input.disabled = true;
   $('#test-listening').hidden = false;
   $('#test-form-row').hidden = true;
+  $('#btn-test-prev').hidden = test.i === 0;
   speakWord(w);
   // Enable input after TTS has had time to start (rough estimate: 1.5s per word)
   const delay = 1500 + (w.definition ? 1500 : 0) + (w.sentence ? 1500 : 0);
@@ -2397,6 +2406,15 @@ function presentTestWord() {
   }, delay);
 }
 
+$('#btn-test-prev').addEventListener('click', () => {
+  if (test.i > 0) {
+    ttsCancel();
+    test.answers.pop();
+    test.i--;
+    presentTestWord();
+  }
+});
+
 $('#btn-test-speak').addEventListener('click', () => speakWord(test.words[test.i]));
 
 $('#test-form').addEventListener('submit', async (e) => {

public/index.html

@@ -245,7 +245,7 @@
 
   <!-- Practice (spelling) -->
   <section id="view-practice" class="view" hidden>
-    <button class="back-btn" data-back="practice">← Stop</button>
+    <button class="back-btn" data-back="practice">← Back</button>
     <p id="practice-progress" class="progress-label"></p>
     <div class="progress-track"><div id="practice-bar" class="progress-fill"></div></div>
 
@@ -265,7 +265,7 @@
 
   <!-- Friday test (spelling) -->
   <section id="view-test" class="view" hidden>
-    <button class="back-btn" data-back="test">← Cancel</button>
+    <button class="back-btn" data-back="test">← Back</button>
     <p id="test-progress" class="progress-label"></p>
     <div class="progress-track"><div id="test-bar" class="progress-fill test-fill"></div></div>
 
@@ -279,6 +279,7 @@
                  autocapitalize="off" autocorrect="off" spellcheck="false" aria-label="Spell the word">
           <button type="submit" class="check-btn">Next →</button>
         </form>
+        <button id="btn-test-prev" class="speak-btn" style="margin-top:.75rem" hidden>← Previous word</button>
       </div>
     </div>