Add dark mode toggle with system preference detection

[MeisnerDan]

Implement a dark mode theme using Tailwind's dark mode capabilities. The app should detect the user's system preference (prefers-color-scheme) and auto-enable dark mode if the system is set to dark. Include a manual toggle in the settings or header to override the system preference, and persist the user's choice in localStorage.

Acceptance Criteria

• Dark mode styles implemented using Tailwind dark: utilities
• System preference detection on app load
• Manual toggle button in header or settings page
• Toggle state persists in localStorage
• All components have proper contrast in both light and dark modes (WCAG AA standard)
• shadcn/ui components work correctly in dark mode
• Dark mode toggle shows current theme state (e.g., sun/moon icon)
• Works across all pages and modals
• No jarring flashes or theme shifts during page load