Add spelling improvements and local Piper TTS

Practice: fire streak badge, first-letter hint on wrong answers, missed-words drill session after practice completes.
Test: audio-only mode with listening state before input appears, larger re-read button.
Words: definition field (DB migration + admin editor + TTS playback).
TTS: replace speechSynthesis with local Piper via /api/tts, WAV cached in data/tts-cache, falls back to browser voice when Piper is not installed.
Dockerfile: switched to node:26-slim, installs Piper binary + en_US-lessac-medium voice.

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

public/admin.html

@@ -258,7 +258,7 @@
             <input type="hidden" id="list-id">
             <input id="list-name" placeholder="List name (e.g. Week of July 6)" aria-label="List name">
             <textarea id="list-words" rows="8"
-              placeholder="One word per line. Add an example sentence with | like:&#10;receive | Did you receive my letter?"></textarea>
+              placeholder="One word per line. Optionally add sentence and definition with | like:&#10;receive | Did you receive my letter? | to get or accept something"></textarea>
             <div class="row-actions">
               <button type="submit">Save list</button>
               <button type="button" id="list-cancel" class="secondary">Clear</button>