feat: Prepare the TUI to allow for custom themes

[unfunco]

Makes consistent the theme configuration so that this can in the future be configured via a config file.

[Copilot]

Pull Request Overview

This PR refactors the theme configuration system to prepare for custom themes, making color handling consistent across both the TUI and CLI help documentation by introducing a unified PaletteColor type that handles conversion between hex colors for Lipgloss and RGBA for Fang.

• Introduced PaletteColor type to unify color representation with methods for both Lipgloss and RGBA conversions
• Added Config struct and parsing functions (FromConfig, MustFromConfig, parsePaletteColor) to support configuration-based theme creation
• Replaced hard-coded ANSI color references with theme-based color references throughout the TUI
• Renamed tabCount to TabCount (exported) for consistency with other exported Tab constants

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
main.go	Updated customColorScheme to use new theme API with RGBA conversion methods, moved function to bottom of file
internal/theme/theme.go	Complete rewrite introducing PaletteColor, Config types, hex color parsing, and new WorryStyle method
internal/theme/theme_test.go	New test file covering color parsing, shorthand hex, invalid input, and ANSI rejection
internal/tui/tui.go	Updated to use new theme API, renamed tabCount to TabCount, removed obsolete comment

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