fix(auth): handle edge-to-edge insets in MFA challenge and auth screens

[demolaf]

Fixes #2403.

The MFA challenge screen had no inset handling, so its title collided with the status bar/camera cutout on edge-to-edge devices. Also found a related, opposite bug in six other auth screens combining Scaffold's innerPadding with a redundant, double-counted safeDrawingPadding() call.

• MfaChallengeDefaults.kt: wrapped DefaultMfaChallengeContent in a Scaffold
• SignInUI, SignUpUI, SignInEmailLinkUI, ResetPasswordUI, EnterPhoneNumberUI, EnterVerificationCodeUI: removed the redundant .safeDrawingPadding(), added a uniform 16.dp margin
• FirebaseAuthScreen.customMethodPickerLayout now renders full-screen (was Scaffold-confined) so fully custom layouts can go edge-to-edge; direct behavior change since library is pre-1.0

Preview

[gemini-code-assist]

Code Review

This pull request refactors the custom method picker layout to render edge-to-edge across the entire screen, adjusts system insets handling by removing safeDrawingPadding from several sub-screens, and fixes a form validation bug in SignUpUI where the sign-up button remained disabled when a display name was not required. A new unit test is also added to verify this fix. The review feedback suggests avoiding the reuse of the high-level modifier parameter inside the NavHost of FirebaseAuthScreen to prevent layout issues during navigation, adding Modifier.fillMaxSize() to the Scaffold in MfaChallengeDefaults.kt to ensure full-screen coverage, and making the entire terms checkbox row clickable in the demo activity to improve accessibility.

[gemini-code-assist]

The high-level modifier parameter of FirebaseAuthScreen should be applied to the root Surface of the screen (at line 198) rather than being passed down to individual child screens inside the NavHost.

Reusing the high-level modifier here is problematic because any modifier styling (such as background, padding, etc.) passed by the caller will only apply to the MethodPicker screen and will suddenly disappear when the user navigates to other screens in the flow (like EmailAuthScreen or PhoneAuthScreen).

The root Surface should be updated to use modifier, and this inner Box should use Modifier.fillMaxSize() instead.

Suggested change

	Box(modifier = modifier.fillMaxSize()) {
	Box(modifier = Modifier.fillMaxSize()) {

[gemini-code-assist]

Similarly, AuthMethodPicker should use a local Modifier rather than reusing the high-level modifier parameter of FirebaseAuthScreen to ensure consistent layout and styling across all screens in the authentication flow.

Suggested change

	modifier = modifier
	.padding(innerPadding),
	modifier = Modifier
	.padding(innerPadding),

[gemini-code-assist]

The Scaffold should be configured with Modifier.fillMaxSize() to ensure it occupies the entire screen area. Without this, the Scaffold's background color and layout might not cover the full screen on some devices or screen configurations, especially when the content is short.

Suggested change

	Scaffold { innerPadding ->
	Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->

[gemini-code-assist]

To improve accessibility and make the touch target larger, the entire Row should be clickable to toggle the checkbox state, rather than requiring the user to tap the small checkbox directly.

We can achieve this by making the Row clickable and setting onCheckedChange to null on the Checkbox so that the click event is handled uniformly by the row.

            Row(
                modifier = Modifier
                    .clickable { onTermsAcceptedChange(!termsAccepted) }
                    .padding(horizontal = 16.dp, vertical = 8.dp),
                verticalAlignment = Alignment.CenterVertically
            ) {
                Checkbox(
                    checked = termsAccepted,
                    onCheckedChange = null
                )