rebrand to useSend

[KMKoushik]

Summary by CodeRabbit

• New Features

  • Optional USESEND_API_KEY, X-Usesend-Email-ID support, and configurable DKIM selector.

• Improvements

  • Bulk send: template caching, domain grouping, and preserved ordering.
  • Default SMTP host/user updated to usesend; UI/SDK rebranded; new typography primitives (H1/H2/BodyText).

• Deprecations

  • SDK rebranded to UseSend; Unsend retained as deprecated alias.

• Documentation

  • Full rebrand across docs, examples, and README.

• Chores

  • Package names, scripts, CI, and image targets migrated to usesend.

• Removal

  • Marketing app simplified; multiple marketing pages/components removed.