Native clients enter beta

Use early native Letter.md clients on iPhone, Android, and macOS with the same Markdown and collaboration model.

Letter.md's native clients now share one document model instead of creating platform-specific copies.

What changed

The iPhone app adds native navigation, library, media, settings, and editor controls.
The Android app uses native Compose screens around the shared Markdown editor.
The macOS app provides a native library and document shell with reviewable AI suggestions.
Authentication tokens stay in Keychain or Keystore and are not passed into the editor WebView.
Previously synchronized work can reopen from device-local recovery data.

Availability remains limited while signing, distribution, and update workflows are completed for each platform.