Improve email display name handling with proper escaping

- Add quote escaping in display names to prevent malformed email headers
- Handle empty string defaultFromName by checking trim()
- Prevent formatting when fromName is only whitespace
- Example: John "The Boss" Doe becomes "John \"The Boss\" Doe" <email>

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

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

src/services/MailingService.ts

@@ -51,7 +51,15 @@ export class MailingService implements IMailingService {
   private getDefaultFrom(): string {
     const fromEmail = this.config.defaultFrom
     const fromName = this.config.defaultFromName
-    return fromName && fromEmail ? `"${fromName}" <${fromEmail}>` : fromEmail || ''
+
+    // Check if fromName exists, is not empty after trimming, and fromEmail exists
+    if (fromName && fromName.trim() && fromEmail) {
+      // Escape quotes in the display name to prevent malformed headers
+      const escapedName = fromName.replace(/"/g, '\\"')
+      return `"${escapedName}" <${fromEmail}>`
+    }
+
+    return fromEmail || ''
   }
 
   private registerHandlebarsHelpers(): void {