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/services/llm.py

@@ -454,6 +454,8 @@ async def build_context(
     workspace_project_id: int | None = None,
     user_timezone: str | None = None,
     conv_id: int | None = None,
+    voice_mode: bool = False,
+    voice_speech_style: str = "conversational",
 ) -> tuple[list[dict], dict]:
     """Build messages array for Ollama with system prompt and context.
 
@@ -516,6 +518,19 @@ async def build_context(
         f"{tool_guidance}"
     ]
 
+    if voice_mode:
+        _style_hints = {
+            "conversational": "Be warm, natural, and conversational — like speaking to a friend.",
+            "concise": "Be brief and to the point. One or two sentences maximum unless detail is essential.",
+            "detailed": "Give thorough, informative responses as if narrating an explanation aloud.",
+        }
+        style_hint = _style_hints.get(voice_speech_style, _style_hints["conversational"])
+        system_parts.insert(0,
+            "VOICE MODE: Respond naturally as if speaking aloud. "
+            "No markdown, bullet points, headers, or code blocks. Complete sentences only. "
+            f"{style_hint}\n\n"
+        )
+
     context_meta: dict = {
         "context_note_id": None,
         "context_note_title": None,