Add color theme support to .dive.yaml config

Problem

Currently, dive has no way to configure TUI colors via the .dive.yaml config file. Users with custom terminal themes (e.g., catppuccin-mocha) may have poor contrast for selected elements.

Proposed Solution

Add a colors section to the .dive.yaml config that allows overriding TUI colors. For example:

colors:
  selected: bright-white
  added: green
  removed: red
  layer: yellow

Use case

Users with dark/custom terminal themes can adjust TUI contrast without needing terminal-level overrides.