feat(auth)!: make each email sign-in mode a real navigation destination

[demolaf]

The multi-step email flow (sign-in, sign-up, reset password, email-link) kept its "which mode" state in local composable state shared across a single destination, so switching modes wiped whatever the user had typed, had no back-stack entry, and never animated. This converts it to one destination per mode, so the typed address survives every switch, system back walks through modes in order, and switches animate with the configured transitions.

AuthRoute gains nested per-step routes for all three auth flows and a routePattern/route distinction, so MfaEnrollmentScreen and PhoneAuthScreen can adopt the same per-step navigation later with no further public API change (tracked as follow-up PRs). Error-recovery navigation is now owned solely by the host — EmailAuthScreen no longer navigates itself on error, only reports.

• EmailAuthDestinations.kt: shared NavGraphBuilder extension registering the four email destinations, used by both FirebaseAuthScreen and the reauthentication sheet.
• EmailAuthScreen: gained mode/onNavigateToMode (paired — passing one without the other throws at composition) and onEmailTyped; onContinueWithProvider removed (the host owns provider hand-off now).
• ReauthPresentationState: route round-trip switched from an order-dependent when to an explicit route-string-to-id map, since a flow and its start step can now share a route string.

Added EmailAuthRouteNavigationTest, FirebaseAuthScreenEmailRecoveryTest, EmailAuthScreenModeSwitchTest, EmailAuthHostDestinationsTest and ReauthPresentationStateSaverTest covering preservation across switches and Activity recreation, back-stack bounding, error-recovery push/replace, and both hosts registering identical destinations.

⚠️ Breaking Changes

• AuthRoute gained nested per-step routes (6 subclasses → 16). An else-less exhaustive when (route) no longer compiles.
• Route string values changed: auth_email → auth_email_signin, auth_phone → auth_phone_number, auth_mfa_enrollment → auth_mfa_enrollment_select_factor.
• EmailAuthScreen.onContinueWithProvider removed.

Usage

EmailAuthScreen(
    ...,
    mode = currentBackStackEntry.mode,      // driven by your own NavHost
    onNavigateToMode = { mode, email, replace -> navController.navigate(...) },
)

---

Maintainer note: Fixes internal CPRN-403

[gemini-code-assist]

Code Review

This pull request refactors the email authentication flow by splitting the single email destination into individual destinations per authentication mode. This preserves the typed email address across mode switches and Activity recreation, and allows proper back-stack navigation. A review comment suggests an improvement in EmailAuthDestinations.kt to perform the redirect navigation atomically in a single transaction rather than calling popBackStack and navigateToEmailStep sequentially.

[russellwheatley]

LGTM - I'll follow up the remaining recovery tests separately