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/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>