Add light theme option

[niklasmarderx]

What

Add a light theme as an alternative to the current dark glassmorphism design.

Why

Some users prefer light themes, especially during daytime. Accessibility-wise, light themes can be easier to read for some people.

How

1. Look at src/styles/variables.css for the CSS custom properties
2. Create a set of light-theme variables (backgrounds, text colors, borders)
3. Add a theme toggle in src/services/ThemeService.ts
4. Use CSS [data-theme='light'] selector to swap variables
5. Add the toggle to the settings modal
6. Persist the preference via StorageService

Reference

The ThemeService already exists — it just needs a light variant.