feat: enhance Unsplash integration with pagination, random images, and portrait orientation
- Added pagination support to search endpoint with page parameter and orientation control (defaults to portrait)
- Implemented new /random endpoint for fetching random portrait images with configurable count
- Enhanced UnsplashPickerComponent with infinite scroll, loading states, and automatic random image loading on open
- Added debounced search input, scroll detection for pagination, and improved error
- Implemented multi-selection for notes with Ctrl+click, long-press, and keyboard shortcuts (Ctrl+A, Escape)
- Added Gemini API integration with environment configuration and routes
- Enhanced code block UI with improved copy feedback animation and visual polish
- Added sort order toggle (asc/desc) for note lists with persistent state
