feat: voice S2S — faster-whisper STT, Kokoro TTS, PTT overlay

Implements full speech-to-speech pipeline (all 4 phases):

Backend (Phase 1):
- services/stt.py: lazy WhisperModel singleton, run_in_executor transcription
- services/tts.py: lazy KPipeline singleton, WAV synthesis at 24kHz/16-bit
- routes/voice.py: /api/voice/status, /voices, /transcribe, /synthesise
- config.py: VOICE_ENABLED, STT_BACKEND, STT_MODEL, TTS_BACKEND env vars
- app.py: load STT/TTS models at startup when VOICE_ENABLED=true
- llm.py: voice_mode + voice_speech_style params inject speak-naturally prefix
- generation_task.py: voice_mode passed through from chat route
- chat.py: "voice" conversation type allowed + excluded from retention cleanup
- pyproject.toml + Dockerfile: faster-whisper, kokoro, soundfile dependencies

Frontend (Phases 2–4):
- composables/useVoiceRecorder.ts: MediaRecorder PTT wrapper
- composables/useVoiceAudio.ts: AudioContext WAV playback wrapper
- BriefingView.vue: Listen button (TTS read-aloud), auto-TTS mode, mic PTT
- VoiceOverlay.vue: global floating PTT button; creates/reuses voice conv;
  full record→transcribe→stream→TTS flow; Space bar hold-to-talk via App.vue
- SettingsView.vue: Voice tab (status badge, speech style, voice/speed)
- App.vue: mounts VoiceOverlay; Space keydown/keyup fires voice:ptt-toggle
- api/client.ts: getVoiceStatus, getVoiceList, transcribeAudio, synthesiseSpeech

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

src/fabledassistant/routes/chat.py

@@ -76,7 +76,7 @@ async def create_conversation_route():
     model = data.get("model", Config.OLLAMA_MODEL)
     conversation_type = data.get("conversation_type", "chat")
     # Only allow known types to prevent accidental misuse
-    if conversation_type not in ("chat", "mcp"):
+    if conversation_type not in ("chat", "mcp", "voice"):
         conversation_type = "chat"
     conv = await create_conversation(uid, title=title, model=model, conversation_type=conversation_type)
     return jsonify(conv.to_dict()), 201
@@ -187,6 +187,7 @@ async def send_message_route(conv_id: int):
         rag_project_id=effective_rag_project_id,
         workspace_project_id=workspace_project_id,
         user_timezone=user_timezone,
+        voice_mode=(conv.conversation_type == "voice"),
     ))
 
     return jsonify({