Implement Software Best Practices and Graceful API Key Management
- Integration of ruff, eslint, prettier for consistent code style
- Global error handling and structured logging in backend
- Comprehensive testing strategy with 100% pass rate (33 backend, 2 frontend)
- Graceful handling of missing Gemini API key (non-blocking startup)
- New /settings/status endpoint and UI warning indicators
- Full documentation update (ARCHITECTURE.md, CONTRIBUTING.md, README.md, GEMINI.md)
- Restored missing frontend logic and fixed UI syntax errors