fix(auth): quick fix for Auth.User sendability error

[ncooke3]

/Users/nickcooke/Developer/quickstart-ios/authentication/AuthenticationExample/ViewControllers/UserViewController.swift:303:25: error: sending 'self.user.some' risks causing data races
        try await user?.delete()
                  ~~~~~~^~~~~~~~
/Users/nickcooke/Developer/quickstart-ios/authentication/AuthenticationExample/ViewControllers/UserViewController.swift:303:25: note: sending main actor-isolated 'self.user.some' to nonisolated instance method 'delete()' risks causing data races between nonisolated and main actor-isolated uses
        try await user?.delete()

[gemini-code-assist]

Code Review

This pull request updates the FirebaseAuth import in UserViewController.swift to use the @preconcurrency attribute, which addresses Swift concurrency warnings. The feedback suggests adding a TODO comment to track the removal of this attribute once the underlying SDK provides Sendable conformance for its types.

[paulb777]

Do we need documentation recommending customers do similar imports?