feat: Add support for light/dark themes

[unfunco]

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[Copilot]

Pull Request Overview

This PR adds support for light and dark theme palettes with automatic detection based on terminal background color. The theme configuration structure has been refactored from a flat structure to separate dark and light palettes with a configurable mode setting.

• Introduced three theme modes: auto (default), dark, and light for controlling palette selection
• Refactored theme initialization to accept a hasDarkBackground parameter for automatic palette selection
• Added comprehensive default palettes optimized for both light and dark backgrounds

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
testdata/config.json	Updated test configuration to include separate dark and light palette definitions
main.go	Modified theme initialization to detect terminal background and pass it to theme functions
internal/theme/theme_test.go	Added test for auto mode light palette selection; updated existing tests for new Config structure
internal/theme/theme.go	Added Mode type, PaletteConfig struct, and logic for palette selection; refactored theme creation to support multiple palettes
internal/config/config_test.go	Updated config tests to validate new nested theme structure
README.md	Added documentation section explaining theme configuration and mode options

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