fix: apply --no-color flag to huh interactive prompts

[srtaalej]

Changelog

Fixed --no-color flag not being applied to interactive prompts (select, confirm, input)

Summary

The --no-color flag stripped styles from CLI output but not from huh interactive prompts (select, confirm, input, etc.), because huh uses its own theme/rendering pipeline. This PR added ThemePlain() — a colorless huh theme with no ANSI styling so that when --no-color is set, forms now use ThemePlain + huh's accessible mode, rendering as plain numbered lists instead of a styled TUI

Test plan

Run slack create --no-color and verify prompts render as plain numbered lists with no color
Run slack login --no-color and verify confirmation prompt has no color
Run slack create (without --no-color) and verify themed prompts still work
go test ./internal/style/ ./internal/iostreams/

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.