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>

Dockerfile

@@ -1,5 +1,22 @@
-FROM node:26-alpine
+FROM node:26-slim
 WORKDIR /app
+
+# Install piper TTS binary and an English voice model
+RUN apt-get update && apt-get install -y --no-install-recommends wget ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN wget -qO /tmp/piper.tar.gz \
+      https://github.com/rhasspy/piper/releases/download/v1.2.0/piper_linux_x86_64.tar.gz \
+    && tar -xzf /tmp/piper.tar.gz -C /tmp \
+    && mv /tmp/piper/piper /usr/local/bin/piper \
+    && rm -rf /tmp/piper*
+
+RUN mkdir -p /app/voices \
+    && wget -qO /app/voices/en_US-lessac-medium.onnx \
+       "https://huggingface.co/rhasspy/piper-voices/resolve/v1.0.0/en/en_US/lessac/medium/en_US-lessac-medium.onnx" \
+    && wget -qO /app/voices/en_US-lessac-medium.onnx.json \
+       "https://huggingface.co/rhasspy/piper-voices/resolve/v1.0.0/en/en_US/lessac/medium/en_US-lessac-medium.onnx.json"
+
 COPY package*.json ./
 RUN npm ci --omit=dev
 COPY . .