chore: Strip unsupported signature formatting by channel (#13046)

# Pull Request Template

## Description

1. This PR is an enhancement to
https://github.com/chatwoot/chatwoot/pull/13045
It strips unsupported formatting from **message signatures** based on
each channel’s formatting capabilities defined in the `FORMATTING`
config

2. Remove usage of plain editor in Compose new conversation modal

Only the following signature elements are considered:
<strong>bold (<code inline="">strong</code>), italic (<code
inline="">em</code>), links (<code inline="">link</code>), images (<code
inline="">image</code>)</strong>.</p>

Any formatting not supported by the target channel is automatically
removed before the signature is appended.

<h3>Channel-wise Signature Formatting Support</h3>

Channel | Keeps in Signature | Strips from Signature
-- | -- | --
Email | bold, italic, links, images | —
WebWidget | bold, italic, links, images | —
API | bold, italic | links, images
WhatsApp | bold, italic | links, images
Telegram | bold, italic, links | images
Facebook | bold, italic | links, images
Instagram | bold, italic | links, images
Line | bold, italic | links, images
SMS | — | everything
Twilio SMS | — | everything
Twitter/X | — | everything


<hr>
<h3>📝 Note</h3>
<blockquote>
<p>Message signatures only support <strong>bold, italic, links, and
images</strong>.<br>
Other formatting options available in the editor (lists, code blocks,
strike-through, etc.) do <strong>not apply</strong> to signatures and
are ignored.</p>
</blockquote>

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Loom video
https://www.loom.com/share/d325ab86ca514c6d8f90dfe72a8928dd


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/constants/editor.js

@@ -210,7 +210,12 @@ export const MARKDOWN_PATTERNS = [
     type: 'em', // PM: em, eg: *italic* or _italic_
     patterns: [
       { pattern: /(?<!\*)\*(?!\*)(.+?)(?<!\*)\*(?!\*)/g, replacement: '$1' },
-      { pattern: /(?<!_)_(?!_)(.+?)(?<!_)_(?!_)/g, replacement: '$1' },
+      // Match _text_ only at word boundaries (whitespace/string start/end)
+      // Preserves underscores in URLs (e.g., https://example.com/path_name) and variable names
+      {
+        pattern: /(?<=^|[\s])_([^_\s][^_]*[^_\s]|[^_\s])_(?=$|[\s])/g,
+        replacement: '$1',
+      },
     ],
   },
   {
@@ -227,11 +232,6 @@ export const MARKDOWN_PATTERNS = [
   },
 ];
 
-export const CHANNEL_WITH_RICH_SIGNATURE = [
-  'Channel::Email',
-  'Channel::WebWidget',
-];
-
 // Editor image resize options for Message Editor
 export const MESSAGE_EDITOR_IMAGE_RESIZES = [
   {