Port backstagephp/mails#86 into the monorepo copy of the package.

- Require Filament authentication and the plugin's canManageMails()
  check on the preview and attachment routes
- Scope attachment lookups to their parent mail so an attachment cannot
  be downloaded through an unrelated mail record
- Return 404 responses for missing or mismatched records
- Render stored email HTML in a sandboxed iframe and add security
  headers to direct preview responses
- Register the routes as authenticated (tenant) panel routes

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>