feat: integrate Sarvam AI transcription and dual-channel system audio capture

[magy070]

Summary

This PR adds support for Sarvam AI Transcription (enabling highly accurate regional Indian language transcription) and implements a dual-channel audio setup on the teacher dashboard to capture both local microphone and desktop system audio (like Zoom/YouTube/Google Meet tab audio).

Key Features Added

1. Sarvam AI Integration:
   • Added backend service to route transcription requests to the Sarvam AI API.
   • Implemented an automatic backend fallback: if the local Whisper transcription server is unreachable, the system seamlessly routes requests to Sarvam AI.
2. Dual-Channel System Audio Capture:
   • Implemented browser-side tab audio capture using Electron's desktop capturer and browser display media API.
   • Created an audio mixing service that combines local mic input and system audio into a single stream for transcription.
3. Audio Setup & Onboarding Wizard:
   • Added a First-Time Audio Setup Wizard to guide teachers in testing and selecting their primary and secondary input devices.
   • Built a System Audio Setup Modal providing instructions on using virtual audio cables on unsupported browsers (Firefox/Safari).

Files Created/Modified

• Backend:
  • backend/src/services/sarvamTranscriptionService.js (New - API wrapper)
  • backend/src/routes/transcription.js (Added provider-specific routes and fallback checks)
  • backend/src/config.js & backend/.env.example (Added Sarvam configuration variables)
• Frontend:
  • frontend/src/services/audioCaptureService.js (New - Dual audio capture logic)
  • frontend/src/components/AudioSetupWizard.jsx (New - Onboarding UI)
  • frontend/src/components/MicDeviceSelector.jsx (New - Device selector dropdowns)
  • frontend/src/components/SystemAudioSetupModal.jsx (New - Setup instruction guide)
  • frontend/src/pages/RoomDetailPage.jsx (Integrated wizard and recording controllers)
  • frontend/src/components/RoomSettingsModal.jsx & frontend/src/services/serverTranscriptionService.js (Integrated settings)