fix: add origin header to auth requests

[AzizOuertani]

Summary

• Adds an origin header (start-ui-native://) to all auth fetch requests in the Expo client
• This prevents a crash in @better-auth/expo@1.5.0 server plugin on Node.js v24, where new Request(request, { headers }) fails with TypeError: Cannot read private member #state
• With the origin header present, the expo plugin's onRequest hook returns early (line 44: if (!expoOrigin) return) and skips the buggy Request cloning code path

Context

The @better-auth/expo server plugin checks for an expo-origin header and, if found, tries to clone the Request object with a new origin header. On Node.js v24, this clone operation fails due to internal undici changes. By sending the origin header directly from the native app, the plugin sees no expo-origin header and exits early.

Alternative: update @better-auth/expo to >=1.5.1 on the web side.

Test plan

• Run the Expo app locally against a Node 24 backend and verify get-session returns 200
• Verify login/signup flows still work correctly
• Verify the web app auth still works (no regression)

🤖 Generated with Claude Code

[github-actions]

🚀 Expo preview is ready!

• Project → start-ui-native-v9iizxkbojzedvpfkfzcq
• Platforms → android, ios
• Scheme → start-ui-native
• Runtime Version → 1.0.0
• More info

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