fix: make STT config env-overridable and fix doc issues

Code fixes:
- STT model, Groq base URL, and OpenAI STT base URL are now
  configurable via env vars (STT_GROQ_MODEL, STT_OPENAI_MODEL,
  GROQ_BASE_URL, STT_OPENAI_BASE_URL) instead of hardcoded
- Gateway and Discord VC now read stt.model from config.yaml
  (previously only CLI did this — gateway always used defaults)

Doc fixes:
- voice-mode.md: move Web UI troubleshooting to web.md (was duplicated)
- voice-mode.md: simplify "How It Works" for end users (remove NaCl,
  DAVE, RTP internals)
- voice-mode.md: clarify STT priority (OpenAI used first if both keys
  set, Groq recommended for free tier)
- voice-mode.md: document new STT env overrides in config reference
- web.md: remove duplicate Quick Start / Step 1-3 sections
- web.md: add mobile HTTPS mic workarounds (moved from voice-mode.md)
- web.md: clarify STT fallback order

gateway/platforms/discord.py

@@ -881,7 +881,18 @@ class DiscordAdapter(BasePlatformAdapter):
             await asyncio.to_thread(VoiceReceiver.pcm_to_wav, pcm_data, wav_path)
 
             from tools.transcription_tools import transcribe_audio
-            result = await asyncio.to_thread(transcribe_audio, wav_path)
+            # Read STT model from config.yaml
+            stt_model = None
+            try:
+                import yaml as _y
+                from pathlib import Path as _P
+                _cfg = _P(os.getenv("HERMES_HOME", _P.home() / ".hermes")) / "config.yaml"
+                if _cfg.exists():
+                    with open(_cfg) as _f:
+                        stt_model = (_y.safe_load(_f) or {}).get("stt", {}).get("model")
+            except Exception:
+                pass
+            result = await asyncio.to_thread(transcribe_audio, wav_path, model=stt_model)
 
             if not result.get("success"):
                 return