feat(voice): add optional speech-to-text input and read-aloud TTS

Adds a push-to-talk mic button in the composer and a read-aloud button on
assistant messages. Both are opt-in and hidden unless a voice backend is
configured via VOICE_SIDECAR_URL.

The auth-gated /api/voice proxy forwards to a configurable backend exposing
/transcribe and /tts (provider-agnostic); the frontend probes /api/voice/health
and hides the controls when disabled. Adds i18n keys and docs/voice.md.

Includes a local, no-API-key reference backend in voice-sidecar/ (faster-whisper
for STT, Kokoro-82M for TTS, both CPU-capable).

.gitignore

@@ -142,3 +142,10 @@ tasks/
 
 # Git worktrees
 .worktrees/
+
+# Voice sidecar (Python) — generated, machine-specific, not committed
+voice-sidecar/.venv/
+voice-sidecar/voice_messages/
+voice-sidecar/**/__pycache__/
+*.pyc
+*.wav