sample(auth): full customization demo

[demolaf]

Extends the full customization demo to supply a custom UI for every FirebaseAuthScreen content slot.

Changes

• Each email mode becomes its own Navigation 3 route instead of a mode swap inside one screen, so the steps get real transitions, a working predictive-back gesture, and an address that survives moving between them.
• Reauthentication renders its own compact confirmation step, because the library composes that step inside a modal bottom sheet where a full sign-in page does not fit. Declining it now reads as a cancellation rather than a failure.
• A returning email link routes back to whichever demo sent it, so several auth demos can sit behind one deep link.

Preview

• Full Customization Flow

1-signin-signup-phone.mp4

• Email Link Sign In Flow

2-email-link-signin.mp4

• Reauthentication Flow

5-reauth.mp4

• MFA Flow (Enrollment)

3-authenticated-mfa-enrollment.mp4

• MFA Flow (Challenge)

4-mfa-challenge.mp4

• 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.

[russellwheatley]

LGTM. Two small non-blocking things worth a look:

• MainActivity.USE_AUTH_EMULATOR flips from true to false — unrelated to this PR's scope, might be intentional but flagging in case it's a leftover from local testing.
• PhoneEntryStep's CTA reads "Sign Up" but the screen is the general phone sign-in entry (title says "Login by phone number"); looks copy-pasted from the email entry step's create-account button.