Mobile app: add OpenAI-compatible and OpenRouter provider support #54
Description
The mobile settings screen (settings_screen.dart) only supports Ollama and Groq as LLM providers. After merging PRs #49 and #51, the web frontend and backend now support OpenAI-compatible and OpenRouter providers.
The mobile app needs to be updated to:
- Add OpenAI-compatible provider option (base URL + optional API key + model selection)
- Add OpenRouter provider option (API key + searchable model dropdown)
- Add independent STT provider selection (Speaches / Groq Whisper) to match the new voice pipeline architecture
The backend API already supports all of this — this is a frontend-only change in the Flutter app.