feat(ai): OpenRouter STT support + real error surfacing + STT endpoint test

- ai.service: route *.openrouter.ai STT to its JSON+base64
  /audio/transcriptions API; keep the OpenAI multipart path (AI SDK) for
  OpenAI/self-hosted whisper. Unify transcription behind transcribe().
- /transcribe controller: surface the real provider/transport reason
  (describeProviderError) instead of an opaque 500; preserve HttpException.
- testConnection: add an 'stt' capability (silent-WAV probe) + DTO; client
  gets a Test endpoint button and status dot on the Voice/STT card.
- useDictation: log full errors to the console and show the real reason
  (mic start + transcription paths); handle NotReadable/Abort and missing
  mediaDevices.
- docs(CLAUDE.md): require full error logging + specific user-facing messages.

apps/client/public/locales/en-US/translation.json

@@ -1189,5 +1189,7 @@
   "No microphone found": "No microphone found",
   "Could not start recording": "Could not start recording",
   "Transcription failed": "Transcription failed",
-  "Voice dictation is not configured": "Voice dictation is not configured"
+  "Voice dictation is not configured": "Voice dictation is not configured",
+  "Microphone is unavailable or already in use": "Microphone is unavailable or already in use",
+  "Audio recording is not available in this browser/context": "Audio recording is not available in this browser/context"
 }