Add cross-browser compatibility for Chrome and Firefox

• Introduced browserAPI detection mechanism for Chrome and Firefox
• Updated background, content, and popup scripts to use cross-browser runtime and storage APIs
• Modified message passing and storage retrieval to support both browser environments
• Bumped extension version to 1.2.0
• Improved error handling and browser-specific implementation details

Refactor extension to use OpenAI for chat, TTS, and note generation

Major changes:
- Replaced ElevenLabs with OpenAI's text-to-speech and chat APIs
- Implemented context-aware page chat with streaming responses
- Added prompt suggestions and enhanced chat UI
- Improved error handling and settings management
- Updated manifest and README to reflect new features