feat: add Groq STT support and fix voice mode keybinding

- Add multi-provider STT support (OpenAI > Groq fallback) in transcription_tools
- Auto-correct model selection when provider doesn't support the configured model
- Change voice record key from Ctrl+Space to Ctrl+R (macOS compatibility)
- Fix duplicate transcript echo in voice pipeline
- Add GROQ_API_KEY to .env.example

.env.example

@@ -275,3 +275,6 @@ WANDB_API_KEY=
 # GITHUB_APP_ID=
 # GITHUB_APP_PRIVATE_KEY_PATH=
 # GITHUB_APP_INSTALLATION_ID=
+
+# Groq API key (free tier — used for Whisper STT in voice mode)
+# GROQ_API_KEY=