feat(auth): add pickerContent and pickerDestination slots to AuthPickerView

[demolaf]

• Related to Customizable SwiftUI auth UI: content slots, per-screen state, and theming #1400

Adds .pickerContent and .pickerDestination to AuthPickerView, so apps can replace the auth sheet's first screen or any pushed screen while the library keeps driving navigation, MFA and account conflicts. Stacked on #1402.

Changes

• AuthPickerView keeps its signature. The new modifiers return AuthPickerContent, which now owns the sheet, NavigationStack, error alert, and MFA and account-conflict handling.
• Add public defaults DefaultAuthPickerContent, DefaultAuthMethodPicker and DefaultAuthPickerDestination, holding the markup that was private inside AuthPickerView.
• .pickerContent replaces only the signed-out screen. SignedInView and the "Authenticating..." overlay stay in AuthPickerContent, so custom content can't hide them.
• DefaultAuthPickerContent is generic over its provider list from the start, so the custom-layout initializer in the next PR is additive.

Naming, pending team review

AuthPickerContent (the wrapper) sits beside DefaultAuthPickerContent (the default for its PickerContent slot). Kept for now; the alternative is renaming the wrapper, e.g. ComposedAuthPickerView, before release.

API Usage

AuthPickerView { authenticatedApp }
  .pickerContent {
    DefaultAuthPickerContent()
      .background(theme.colors.background)
  }
  .pickerDestination { screen in
    switch screen {
    case .enterPhoneNumber:
      MyPhoneEntry()
    default:
      DefaultAuthPickerDestination(screen: screen)
        .background(theme.colors.background)
    }
  }

Preview

---

Maintainer note: Fixes internal CPRN-516

[gemini-code-assist]

Code Review

This pull request refactors the authentication UI flow in FirebaseAuthSwiftUI by extracting the sheet presentation, navigation stack, and error/MFA handling into a new AuthPickerContent view. This allows AuthPickerView to support customizable first screens and destination views via pickerContent and pickerDestination modifiers, while falling back to the newly introduced DefaultAuthPickerContent and DefaultAuthPickerDestination. Key feedback points out that DefaultAuthPickerContent's generic parameter is overly restricted by its initializer, a UIKit dependency is introduced via UIColor.label, and a redundant Task dispatch is used in reportError on a MainActor-annotated struct.

[gemini-code-assist]

The DefaultAuthPickerContent struct is generic over AuthMethodPicker, but the only initializer provided restricts AuthMethodPicker to DefaultAuthMethodPicker. This prevents callers from ever providing a custom AuthMethodPicker view, which defeats the purpose of the generic parameter.

To allow callers to customize the provider buttons list while keeping the default logo, email sign-in, and terms, please add a public initializer that accepts a custom @ViewBuilder closure for AuthMethodPicker.

Suggested change

	public struct DefaultAuthPickerContent<AuthMethodPicker: View>: View {
	public init() where AuthMethodPicker == DefaultAuthMethodPicker {
	authMethodPicker = { DefaultAuthMethodPicker() }
	}
	public struct DefaultAuthPickerContent<AuthMethodPicker: View>: View {
	public init(@ViewBuilder authMethodPicker: @escaping () -> AuthMethodPicker) {
	self.authMethodPicker = authMethodPicker
	}
	public init() where AuthMethodPicker == DefaultAuthMethodPicker {
	self.authMethodPicker = { DefaultAuthMethodPicker() }
	}

[gemini-code-assist]

Using Color(UIColor.label) introduces a dependency on UIKit (via UIColor), which can cause compilation issues on non-iOS platforms (like macOS) and relies on transitive imports since UIKit is not explicitly imported in this file.

Since this is a SwiftUI view, you can use the platform-agnostic Color.primary instead, which automatically adapts to light and dark modes.

Suggested change

	Image(systemName: "xmark")
	.foregroundStyle(Color(UIColor.label))
	}
	Image(systemName: "xmark")
	.foregroundStyle(Color.primary)
	}

[gemini-code-assist]

Since AuthPickerContent is annotated with @MainActor, the reportError method is already guaranteed to run on the main actor. Spawning a new asynchronous Task to update self.error is redundant and introduces an unnecessary dispatch hop. You can update the state directly.

  private func reportError(_ error: Error) {
    self.error = AlertError(
      message: authService.string.localizedErrorMessage(for: error),
      underlyingError: error
    )
  }