Eliminate code duplication in email sanitization

- Create centralized sanitization utilities in utils/helpers.ts
- Add sanitizeDisplayName() with configurable quote escaping
- Add sanitizeFromName() wrapper for consistent fromName handling
- Replace duplicated sanitization logic in sendEmail.ts (9 lines → 1 line)
- Replace duplicated sanitization logic in MailingService.ts (9 lines → 1 line)
- Export new utilities from main index for external use
- Maintain identical functionality while reducing maintenance overhead

Benefits:
- Single source of truth for email header sanitization
- Consistent security handling across all email components
- Easier to maintain and update sanitization logic
- Configurable quote escaping for different use cases

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/services/MailingService.ts

@@ -7,6 +7,7 @@ import {
   BaseEmail, BaseEmailTemplate, BaseEmailDocument, BaseEmailTemplateDocument
 } from '../types/index.js'
 import { serializeRichTextToHTML, serializeRichTextToText } from '../utils/richTextSerializer.js'
+import { sanitizeDisplayName } from '../utils/helpers.js'
 
 export class MailingService implements IMailingService {
   public payload: Payload
@@ -44,17 +45,10 @@ export class MailingService implements IMailingService {
 
   /**
    * Sanitizes a display name for use in email headers to prevent header injection
-   * and ensure proper formatting
+   * Uses the centralized sanitization utility with quote escaping for headers
    */
   private sanitizeDisplayName(name: string): string {
-    return name
-      .trim()
-      // Remove/replace newlines and carriage returns to prevent header injection
-      .replace(/[\r\n]/g, ' ')
-      // Remove control characters (except space and printable characters)
-      .replace(/[\x00-\x1F\x7F-\x9F]/g, '')
-      // Escape quotes to prevent malformed headers
-      .replace(/"/g, '\\"')
+    return sanitizeDisplayName(name, true) // escapeQuotes = true for email headers
   }
 
   /**