Show resolved theme details from the CLI

[jongio]

Theme authors can list themes, but they cannot inspect the resolved palette without launching the TUI or reading TOML by hand. Add grut theme show <name> --json to print the loaded theme metadata and color slots.

Acceptance criteria:

• grut theme show default --json prints theme name, variant, mode, and colors.
• Unknown theme names follow the existing theme loader fallback behavior.
• The existing grut theme list behavior stays unchanged.