sample(auth): full customization demo - #2414

Extends the full customization demo so the sample supplies a custom UI for every FirebaseAuthScreen slot: emailContent, phoneContent, mfaEnrollmentContent (factor choice, SMS, TOTP, verification), mfaChallengeContent, reauthContent and authenticatedContent.

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

• screens/AuthMethodPickerUI.kt hosts the primary email flow on its own Navigation 3 back stack — one route per EmailAuthMode plus the demo's own EmailEntryKey — instead of swapping state.mode inside a single screen. That is what gives the email steps real transitions, a working predictive-back gesture, and an address that survives moving between them.
• screens/reauth/ is two screens. ReauthUI.kt is the provider chooser (ReauthContentState.providers arrives already filtered to the providers linked to this user). ReauthEmailStep.kt is a compact "Confirm it's you", because the library composes the reauth email step inside a ModalBottomSheet — the full sign-in page does not fit there, and its sign-up and email-link affordances are switched off in reauth anyway.
• MainActivity.kt routes a returning email link back to whichever demo sent it, keyed on a path segment each demo puts on its continue URL. Path rather than query: ContinueUrlBuilder appends ? unconditionally, which would corrupt an existing query string. FullCustomizationDemoActivity now consumes EXTRA_EMAIL_LINK so the link can complete there.
• A declined reauthentication is treated as a cancellation rather than a failure, following the withReauth contract in this stack — backing out of the sheet no longer reports "couldn't change the password".
• common/AuthPage.kt: shared page frame (mascot, headline, card, bottom actions) for the MFA, reauth and authenticated screens, so their background, scrolling and inset behaviour match the email and phone steps.
• Also addresses review feedback on the existing screens: 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

• Email Link Sign In Flow

• Reauthentication Flow

• MFA Flow (Enrollment)

• MFA Flow (Challenge)

• Error Dialog (Inherits font change)