Add dark/light theme toggle for better accessibility

[shivamshrma09]

The dark theme is nice but sometimes I need light mode, especially during daytime.

Current issue:

• Only dark theme available
• Hard to use in bright environments
• No user preference saving

What would help:

• Toggle button to switch themes
• Remember my preference
• Light theme that's easy on eyes
• Smooth transition between themes

Some people prefer light themes and this would make the app more accessible.