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 71.13%. Comparing base (a6658e3) to head (e9a8f33).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #454      +/-   ##
==========================================
+ Coverage   71.08%   71.13%   +0.04%     
==========================================
  Files         220      220              
  Lines       18512    18547      +35     
==========================================
+ Hits        13160    13194      +34     
+ Misses       4177     4176       -1     
- Partials     1175     1177       +2     

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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

[zimeg]

@srtaalej LGTM! And thanks for exploring what's possible with these packages 📺 ✨

I'm approving this after a bit more testing and some changes noted below. If this all looks alright I think we can merge it! One note we might want to follow up with support for the variables:

$ NO_COLOR=1 slack run

Which right now removes format from huh but not the select options! No blocker for this PR 🚢

I hope the note on ANSI limitations makes sense too 📚 We might find that unicode and other box drawing characters are alright but not escaped sequences for this flag 🤓

[zimeg]

🌟 praise: This is a clever approach @srtaalej!

👾 note: I made changes in 2c51483 that adjust this to remove the bold and underline formatting when using this --no-color flag. We'll want to avoid ANSI altogether:

🎨 https://en.wikipedia.org/wiki/ANSI_escape_code#Select_Graphic_Rendition_parameters

📸 note: Here's a move recent preview, with also changes of 784f9f2 added:

delete.mov