docs: getting started for firebaseUI auth iOS for SwiftUI

[russellwheatley]

• Wrote a getting started document for the latest FirebaseUI iOS SwiftUI to mirror closely the previous: https://firebase.google.com/docs/auth/ios/firebaseui

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new, dedicated getting started guide for FirebaseUI authentication tailored for SwiftUI applications. The document provides developers with clear, step-by-step instructions to integrate and configure various authentication methods, enabling them to quickly implement modern, SwiftUI-first sign-in experiences.

Highlights

• New SwiftUI Getting Started Guide: A comprehensive guide for integrating FirebaseUI authentication with SwiftUI applications has been added, covering setup, various sign-in methods, and customization options.
• Swift Package Manager Integration: Detailed instructions are provided for adding FirebaseUI for SwiftUI via Swift Package Manager, including core and provider-specific packages.
• Multi-Provider Authentication Setup: The guide outlines how to configure and register multiple authentication providers such as Email/Password, Apple, Google, Facebook, Phone, Twitter, and generic OAuth/OIDC.
• SwiftUI-First Sign-In Flows: Developers are guided on using AuthPickerView for opinionated default UI flows and AuthService for more custom authentication experiences.

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[gemini-code-assist]

Code Review

The pull request introduces a new GETTING_STARTED.md documentation file for FirebaseUI for SwiftUI, detailing setup, authentication provider configuration, and usage examples. Review comments identify several issues and improvement opportunities within the provided Swift code snippets: a critical compilation error in the AppDelegate's application(_:open:options:) function due to a missing return statement, a potential runtime crash from force-unwrapping Bundle.main.bundleIdentifier, and a hardcoded APNS token type (.prod) that should ideally be conditional (.sandbox for debug builds). Additionally, the review suggests consolidating duplicated and incomplete AppDelegate examples to improve clarity and prevent confusion.

[gemini-code-assist]

This function is declared to return a Bool, but it's missing a return statement. This will cause a compilation error. You should return the boolean result from Auth.auth().canHandle(url).

Suggested change

	Auth.auth().canHandle(url)
	return Auth.auth().canHandle(url)

[gemini-code-assist]

Force-unwrapping Bundle.main.bundleIdentifier with ! can cause a runtime crash if the value is nil. To promote safer coding practices in this guide, it's better to handle this optional gracefully. For example, you could use guard let to safely unwrap it and provide a clear error message if it's missing.

[gemini-code-assist]

The code hardcodes the APNS token type to .prod, which is intended for production builds submitted to the App Store. During development and testing, the token type should be .sandbox. Using compiler directives to switch between the two based on the build configuration is a common practice and would make this example more robust for developers.

Suggested change

	Auth.auth().setAPNSToken(deviceToken, type: .prod)
	#if DEBUG
	Auth.auth().setAPNSToken(deviceToken, type: .sandbox)
	#else
	Auth.auth().setAPNSToken(deviceToken, type: .prod)
	#endif

[gemini-code-assist]

This section provides a more complete implementation for handling provider callbacks, which is great. However, a different and incomplete version is provided earlier under the 'Phone number' section (lines 177-184), which also contains a compilation error. This duplication can be confusing. To improve clarity, consider consolidating the AppDelegate examples into a single, comprehensive section that developers can refer to for all provider-related URL handling.