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

[srtaalej]

Changelog

N/A - Fixes for unreleased changes.

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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.42%. Comparing base (15e0c40) to head (1327317).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #454      +/-   ##
==========================================
- Coverage   70.42%   70.42%   -0.01%     
==========================================
  Files         221      221              
  Lines       18539    18568      +29     
==========================================
+ Hits        13057    13076      +19     
- Misses       4307     4314       +7     
- Partials     1175     1178       +3     

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