fix(auth): allow customizing top app bar colors via AuthUITheme

[demolaf]

Fixes #2390.

AuthUITheme.topAppBarColors was a companion-object getter derived from the ambient MaterialTheme.colorScheme, so there was no way to override it per AuthUITheme instance — apps couldn't align the auth screens' top app bar with their own branding.

Usage

val customTheme = AuthUITheme.Default.copy(
    topAppBarColors = TopAppBarColors(
        containerColor = Color(0xFFFFA000), // firebase primary color - amber
        scrolledContainerColor = Color(0xFFFFA000),
        navigationIconContentColor = Color.White,
        titleContentColor = Color.White,
        actionIconContentColor = Color.White,
        subtitleContentColor = Color.White,
    )
)

val configuration = authUIConfiguration {
    context = applicationContext
    theme = customTheme
    // ...
}

Preview

[gemini-code-assist]

Code Review

This pull request introduces the ability to customize top app bar colors on authentication screens by adding a topAppBarColors property to AuthUITheme and applying it across various UI screens. The review feedback highlights a compilation error in HighLevelApiDemoActivity.kt due to the use of a non-existent subtitleContentColor parameter in Material 3's TopAppBarColors. Additionally, because TopAppBarColors is an experimental API, the reviewer recommends adding the @OptIn(ExperimentalMaterial3Api::class) annotation locally in the demo activity and propagating the @property:ExperimentalMaterial3Api annotation to the public property in AuthUITheme.

[russellwheatley]

A few bits of feedback

SelectFactorUI's TopAppBar in MfaEnrollmentDefaults.kt doesn't take colors at all, so it won't pick up topAppBarColors (or even the existing default). Can you wire that one up too, otherwise the 2FA screen won't match once someone sets a custom color.

see auth/src/main/java/com/firebase/ui/auth/ui/screens/MfaEnrollmentDefaults.kt:255 (the TopAppBar call inside SelectFactorUI)

[russellwheatley]

LocalAuthUITheme.current.topAppBarColors ?: AuthUITheme.topAppBarColors is copied into all 6 screen files - can we pull that into one resolved property on AuthUITheme instead? Saves updating 6 files next time this logic changes.

[russellwheatley]

The new tests just re-check the theme object, not the actual screens - since the fallback's duplicated everywhere, this won't catch a screen missing it. Worth testing off the shared helper once it's extracted (see point 1)