feat: add dark mode support across the application

Overview

Add dark mode support to improve user experience in low-light environments and provide theme preference flexibility.

Problem

Currently, the application only supports a light theme. Users who prefer dark interfaces may experience eye strain, especially during extended usage.

Proposed Solution

• Implement a dark theme color palette.
• Add a theme toggle (Light / Dark / System default).
• Persist user preference in local storage.
• Ensure all components (buttons, cards, inputs, modals, etc.) support both themes.
• Verify accessibility contrast ratios.

Technical Considerations

• Use theme provider / context for global theme management.
• Avoid hardcoded colors.
• Follow consistent design tokens.
• Test responsiveness and UI consistency in both themes.

Acceptance Criteria

• Dark mode toggle is functional.
• Theme preference is saved.
• No UI contrast or visibility issues.
• Fully responsive across devices.

Additional Context

Dark mode should align with the current design system and maintain a minimal, modern aesthetic.