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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[zimeg]

@srtaalej Super nice approach to bringing a more... "boring?" theme to forms 🎨 ✨

I'm leaving an issue we might find quick to address around confirm defaults before merging. I'm also curious if the hints below the prompts can have formatting removed or if that's internal upstream?

←/→ toggle • enter submit • y Yes • n No

[zimeg]

📸 issue: I'm not sure if these are the correct options but I find the confirm default isn't highlighted now. I'll include a screenshot for other reviewers:

[zimeg]

👾 ramble: I'm now uncertain if the comment about "hint" formatting is too nuanced but I do find that the Charm team offers a "default" theme that might be useful to use here for confidence in edge cases?

🔗 https://github.com/charmbracelet/huh?tab=readme-ov-file#themes