Flutter Native Google Auth (NSU)

Prompt file imported from CodeCraftsmaniac/CSE226-Project-2 (.github/prompts/flutter-native-google-auth.prompt.md). Copyright stays with the author.

Implement native Google-only authentication for this workspace's flutter client.

Scope:

• Edit only flutter_app unless a required configuration dependency is missing.
• Do not modify CLI, web_app, or backend auth flows unless explicitly required.

Reference implementation:

• nsu_nexus_app Google auth service

Requirements:

1. Use native Google account picker UX (Perplexity-style), not browser/device-code flow.
2. Keep Google as the only sign-in method.
3. Mirror the reference flow:
   • GoogleSignIn(serverClientId: ...)
   • retrieve idToken (and access token when available)
   • exchange with Supabase via signInWithIdToken
   • persist session token and user profile
   • support full logout (local session + Google sign-out)
4. Reuse existing package/config where possible (google-services.json, package id, OAuth credentials).
5. Ensure all auth-consuming screens use one consistent auth service/state.
6. Preserve existing app styling and navigation behavior.
7. Provide friendly errors for:
   • config error (ApiException: 10 / DEVELOPER_ERROR)
   • cancelled sign-in
   • network/auth backend failures
8. Run static checks on touched files and resolve new compile issues.

Response format:

• Files changed
• Why each change was made
• Verification commands run and result
• Manual setup still required (if any), especially SHA-1 / OAuth client setup