Add configurable color themes (dark, light, solarized)

[rohitgs28]

Summary

Allow users to switch between color themes via config.toml or a keybinding.

Scope

• Ship 3 built-in themes: dark (current default), light, solarized
• Ctrl+T cycles through themes
• theme = "dark" in config.toml
• All colors in renderer.rs should reference theme constants, not hardcoded Color values

Implementation

1. Create src/ui/theme.rs with a Theme struct containing all color definitions
2. Add theme field to App struct
3. Update renderer.rs to use theme colors
4. Add theme to AppConfig

This is a great first contribution for someone who wants to learn ratatui styling.