fix(auth): expose slot parameters through FirebaseAuthScreen

[demolaf]

Closes #2327 .

FirebaseAuthScreen accepted customization slots on sub-screens (custom method picker layout, email/phone/MFA content) but never forwarded them, making them unreachable from the high-level API. This adds the missing parameters and wires them through.

Also refactors the demo app's Custom Slots & Theming screen into separate activities, adds a SpotlightMethodPicker demo for customMethodPickerLayout, and adds unit tests verifying each slot is forwarded correctly.

Configurable params

@Composable
fun FirebaseAuthScreen(
    configuration: AuthUIConfiguration,
    onSignInSuccess: (AuthResult) -> Unit,
    onSignInFailure: (AuthException) -> Unit,
    onSignInCancelled: () -> Unit,
    modifier: Modifier = Modifier,
    authUI: FirebaseAuthUI = FirebaseAuthUI.getInstance(),
    emailLink: String? = null,
    mfaConfiguration: MfaConfiguration = MfaConfiguration(),

    // New
    customMethodPickerLayout: (@Composable (List<AuthProvider>, (AuthProvider) -> Unit) -> Unit)? = null,
    emailContent: (@Composable (EmailAuthContentState) -> Unit)? = null,
    phoneContent: (@Composable (PhoneAuthContentState) -> Unit)? = null,
    mfaEnrollmentContent: (@Composable (MfaEnrollmentContentState) -> Unit)? = null,
    mfaChallengeContent: (@Composable (MfaChallengeContentState) -> Unit)? = null,

    authenticatedContent: (@Composable (state: AuthState, uiContext: AuthSuccessUiContext) -> Unit)? = null,
) {

}

Expected behavior

custom-slots-fix.mp4

[gemini-code-assist]

Code Review

This pull request introduces comprehensive UI customization capabilities to FirebaseAuthScreen by adding slot parameters for email, phone, MFA enrollment, and MFA challenge screens, as well as a custom method picker layout. The demo application has been refactored into separate activities to showcase these new features, including custom method pickers and shape customization. Review feedback suggests optimizing provider categorization using groupBy in the demo, ensuring CircularProgressIndicator uses Modifier.size() to prevent stretching and visual jumps, and reordering function parameters in AuthMethodPicker to place the lambda at the end for better Kotlin idiomaticity.

[mikehardy]

LGTM - but an LGTM comment with no details looks like a rubber stamp so I'll attest that I scanned through the whole thing with actual eyes and it looks really good. Burnt a few tokens on it as well and there were some nitpicks (there are always nitpicks) but nothing that would meet cost/benefit of even mentioning. TL;DR: looks like great work, +1