feat: add configurable light theme support

[lukaszcz]

Add configurable light theme support with automatic background detection. Improves readability on light terminals across the diff viewer, file tree, footer, and search highlights.

• New Features

  • UI.theme supports "auto" (default), "light", and "dark"; DIFFNAV_THEME env var overrides config.
  • Auto-detects terminal background when theme is "auto" and updates styles live.
  • Light theme passes delta args for high-contrast diffs (Monokai Extended Light, readable +/- backgrounds, line number styles).
  • Selection and accent colors adapt to background for file tree, footer, and search results.

• Migration

  • Optional: set UI.theme in config to "auto", "light", or "dark".
  • Optional: set DIFFNAV_THEME to force a theme regardless of config.