feat: Real-time transcription

## What
Implement real-time transcription during recording instead of post-processing.

## Why
- Current: Transcription only starts after recording stops
- Blocks: Live captioning or real-time meeting assistance features
- Would enable live captions and immediate feedback
- Competitive feature compared to other transcription apps

## How
- Use Whisper streaming mode or chunked transcription
- Process audio chunks during recording
- Display partial transcripts in UI
- Handle trade-off between latency and accuracy