feat: user configuration system with CLI flags

[HerbHall]

Description

Expand the settings system to support persistent user configuration with CLI flag overrides. The config file (JSON or TOML) stores preferences; CLI flags override them per-session.

Configuration Options

• Menu mode: flat list (default) vs category drill-down
• Disabled games: list of games to hide from the menu (favorites/shortlist mode)
• Transition effects: on/off toggle, effect selection, random mode
• Digital rain timer: transition duration/speed
• Color theme: effect color (green default), preset colors, random color mode
• Color palette: which colors are included in random rotation

CLI Flags

cli-play --menu=flat|categories
cli-play --no-transitions
cli-play --color=green|cyan|gold|random
cli-play --disable=snake,tetris

Implementation Notes

• Build on existing internal/settings package
• Config file at ~/.config/cli-play/config.json (or XDG standard)
• CLI flags via pflag or stdlib flag
• Settings UI (existing) should expose all options
• Validate that disabled games don't break menu indices

Dependencies

None -- this is a foundational feature that other issues depend on.