fix: Block inline images in message signatures (#13772)

# Pull Request Template

## Description

This PR includes, block inline images in message signatures and prevent
auto signature insertion when editor is disabled.

- Strip inline base64 images from signature on save and show warning
message
- Add `INLINE_IMAGE_WARNING` translation key for signature inline image
removal notification
- Add disabled check to `addSignature()` to prevent signature insertion
when editor is disabled
- Add `isEditorDisabled` checks to signature toggle logic in
`toggleSignatureForDraft()`, `replaceText()`, and `clearMessage()`
- Remove unused `replaceText` from the codebase, which belongs to old
`textarea` editor

Fixes
https://linear.app/chatwoot/issue/CW-6588/the-browser-hangs-when-the-message-signature-contains-inline-image

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

### Loom video
https://www.loom.com/share/fb556b46a12a4308a737eed732d5ed73


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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
- [ ] 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/helper/editorHelper.js

@@ -32,6 +32,25 @@ export function extractTextFromMarkdown(markdown) {
     .trim(); // Trim any extra space
 }
 
+/**
+ * Removes inline base64 markdown images from signature content.
+ *
+ * @param {string} content
+ * @returns {{ sanitizedContent: string, hasInlineImages: boolean }}
+ */
+export function stripInlineBase64Images(content) {
+  if (!content || typeof content !== 'string') {
+    return { sanitizedContent: content || '', hasInlineImages: false };
+  }
+
+  const markdownInlineBase64ImageRegex =
+    /!\[[^\]]*]\(\s*data:image\/[a-zA-Z0-9.+-]+;base64,[^)]+\s*\)/gi;
+  const sanitizedContent = content.replace(markdownInlineBase64ImageRegex, '');
+  const hasInlineImages = sanitizedContent !== content;
+
+  return { sanitizedContent, hasInlineImages };
+}
+
 /**
  * Strip unsupported markdown formatting based on channel capabilities.
  * Uses MARKDOWN_PATTERNS from editor constants.