[FEATURE] Add dark mode toggle

[devmaster1987]

Feature Description

Add a dark mode option to the Dynamic Family Tree application.

Users should be able to switch between the current light theme and a dark theme based on their preference.

Problem

Currently, the application only supports a light theme.

Some users may prefer a darker interface for better readability in low-light environments.

Proposed Solution

Implement a dark mode toggle button in the navigation bar.

The feature should:

• Add a toggle button in the header/navbar.
• Switch the complete interface between light and dark themes.
• Save the user's preference using LocalStorage.
• Keep the preference after refreshing the page.

Expected Behavior

After clicking the dark mode toggle:

• Background colors should change.
• Text colors should remain readable.
• Cards, buttons, modals, and sections should update properly.
• User preference should persist after page refresh.

Acceptance Criteria

• Dark mode toggle button is visible.
• User can switch between light and dark themes.
• Theme preference is saved in LocalStorage.
• Theme remains after browser refresh.
• All sections support dark mode.
• No existing functionality is affected.

Additional Information

This issue is suitable for beginner contributors.

Suggested technologies:

• CSS variables
• JavaScript DOM manipulation
• LocalStorage

No external libraries should be added.