[Feature] Color themes for terminal output via `pv config

[kumar-ayan]

Let users pick their preferred color scheme for all CLI output.

What to build:

• Add theme option to pv config: default, minimal, high-contrast
• default → current colors (green/yellow/red scores)
• minimal → no colors, plain text only
• high-contrast → bolder colors for accessibility
• Store in config.json as { theme: "default" }
• Apply theme across all chalk usage via a central theme helper

Files to touch: src/utils/config.ts, src/ui/ (new theme.ts)