Add Internal note indicator to email notifications

What does this MR do and why?

Add a visible 'Internal note' indicator to email notifications triggered by internal notes (note.internal? == true). Previously, internal notes sent via email looked identical to public comments, causing confusion about visibility and concern that confidential information is being leaked when the comments are internal only.

• HTML email: renders a styled amber badge (🔒 Internal note) above the author line, using inline styles for email client compatibility
• Plain-text email: prepends [Internal note] on its own line before the note body
• Mailer preview: adds note_issue_email_for_internal_note preview to NotifyPreview
• Specs: covers HTML and plain-text variants for internal notes, plus regression guard for public comments

Part of #487396

References

• Closes #606746 (closed)
• Parent issue: #487396

Screenshots or screen recordings

Before	After
Internal note email looks identical to public comment	Email shows amber 'Internal note' badge above author line

How to set up and validate locally

1. In rails console, create an internal note on an issue:

   project = Project.first
   user = User.first
   issue = project.issues.first
   note = Notes::CreateService.new(project, user, noteable_type: 'Issue', noteable_id: issue.id, note: 'This is internal', internal: true).execute
   Notify.note_issue_email(user.id, note.id).deliver_now

2. Check the email in your mail catcher — it should show the amber 'Internal note' badge
3. Run bundle exec rspec spec/mailers/notify_spec.rb -e 'internal' to verify specs pass

MR acceptance checklist

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