Feature: Add dark/light theme toggle

## Description
Allow users to switch between dark and light themes based on preference.

## Requirements
- [ ] Light theme design
- [ ] Dark theme design (current)
- [ ] Theme toggle in settings/header
- [ ] Persist theme preference in localStorage
- [ ] Respect system preference by default
- [ ] Smooth transition animation

## Labels
`enhancement`, `frontend`, `ui/ux`, `good first issue`