Add BetterCaptions plugin for live transcription in Slack

[BittuBarnwal7479]

Summary:

• Adds system-audio captions using OpenAI, xAI, or custom Whisper-compatible APIs.
• Includes configurable language, models, segment interval, and Australian inversion mode.
• Handles capture lifecycle, request timeouts, and safe Electron-to-renderer communication.

fixes: #128

[BittuBarnwal7479]

Hey @3kh0 , I don’t have an OpenAI API key, so I couldn’t test against the live API. Remaining flow has been tested with mocked API responses, including normal and Australian modes.

[BittuBarnwal7479]