feat: make gradient colours configurable (#133)

[linkzera]

Add configurable gradient support for progress bar

This PR introduces a new --gradient (-g) flag that allows users to customize the progress bar gradient. Users can now select from predefined presets (default, sunset, aqua, forest, fire) or specify two custom hex colors separated by a comma (e.g., --gradient=#00FF00,#0000FF). If the flag is not set or an invalid value is provided, the default gradient is used.

Features:

New --gradient flag for selecting gradient style. Support for both presets and custom color pairs. Updated help text to document the new flag.