feat: add React Native support and improve agent components

[barney-w]

Summary

• Add React Native platform components (.native.tsx) for chat, response, and core packages using NativeWind v4 and platform file extensions
• Add native ThemeProvider, token builds for React Native, and Expo mobile app workspace
• Add file attachment support to MessageComposer (drag-and-drop, paste, file picker) with Attachment type and preview rendering in MessageBubble
• Add bodyExtra and streamAdapter config options to useAgentChat for custom request fields and React Native SSE transport
• Improve dark mode token contrast (accent, status colours), markdown rendering (italics, horizontal rules, ordered lists), SourceCard link support, and streaming cursor behaviour
• Fix useCharacterDrain whitespace skipping to prevent markdown structure jumps during character drain