[FEATURE] Add Smooth Theme Transition Animation

[Rucha0901]

Summary

Improve the dark/light theme switching experience by adding smooth transition animations.

Contexts

The current theme switch changes abruptly, which feels visually inconsistent with the modern UI design.

Tasks

• Add transition animations for theme changes
• Animate background and text color updates
• Add icon transition effects
• Optimize transition performance

Acceptance Criteria

• Theme transitions occur smoothly
• No flickering during theme switch
• Animation performance remains stable
• UI consistency is maintained

Area

web

Difficulty

medium

[Rucha0901]

/assign under gssoc'26