sample(auth): full customization demo

[demolaf]

Extends the full customization demo to the remaining FirebaseAuthScreen slots, so the sample now supplies a custom UI for every one: mfaEnrollmentContent (all five enrollment steps), mfaChallengeContent, reauthContent and authenticatedContent.

The authenticated screen mostly exists to make the other two reachable — "Set up two-factor" navigates into enrollment, and changing the password runs through authUI.withReauth, which is what provokes the reauth slot.

• common/AuthPage.kt: shared page frame (mascot, headline, card, bottom actions) for the new screens, so their background, scrolling and inset behaviour match the existing email and phone steps.
• screens/reauth/ReauthUI.kt handles password reauthentication only, with an explicit fallback for accounts that have no password provider — the federated path the default reauth sheet uses is internal/private, so it isn't expressible from a custom slot.
• Also addresses review feedback on the existing screens: system back returns to email entry from the login/sign-up steps, the other-sign-in-methods sheet scrolls (nine providers plus the ToS footer overflowed shorter screens), and the confirmation email is compared trimmed and case-insensitively.

Preview

• Full Customization Flow

full-customization-flow.webm

• Other sign in methods via bottom sheet

other-sign-in-methods.webm

• Error Dialog (Inherits font change)

[gemini-code-assist]

Code Review

This pull request introduces a fully customized authentication flow demo (FullCustomizationDemoActivity and associated steps) to showcase the capabilities of customMethodPickerLayout in FirebaseAuthScreen, alongside minor inset adjustments in the default auth screens. The review feedback is highly constructive and should be addressed: it recommends implementing a BackHandler for proper step-back navigation, making the alternative sign-in methods sheet scrollable to prevent clipping on smaller screens, passing the FirebaseAuth instance dynamically to support custom configurations, performing case-insensitive and trimmed email validation, and ensuring the loading indicator in CtaButton has sufficient contrast.