Dark mode for the app

Users need a dark theme toggle so the app is easier on the eyes at night and saves battery on OLED devices. Request: Add a Dark Mode with an option to follow the OS/system theme (iOS/Android/macOS/Windows) and a manual toggle in Settings > Display. This should include themed UI components and accessible contrast levels. Benefit: Less eye strain during late-night work and improved battery life on OLED devices (e.g., developers and customer-support teams working outside business hours).