feat(auth)!: require the multi-step auth screens to be given navigation

[demolaf]

EmailAuthScreen, PhoneAuthScreen and MfaEnrollmentScreen took their navigation parameters as nullable and fell back to internal localMode/localStep state when they were omitted. That fallback had no back stack and no BackHandler, so system back did nothing and the in-UI switch controls were the only way to move between steps — while the KDoc described those same parameters as a supported way to drive the screens from outside.

The parameters are now required and the fallback is gone, so every caller brings real navigation. Nothing new was published to support that: both slot demos were rewritten to host the screens on their own Navigation 3 back stacks using only existing public API, which is what makes the documented capability real rather than implied.

• EmailAuthSlotDemoActivity.kt / PhoneAuthSlotDemoActivity.kt: each defines its own NavKey and back stack, so system back steps between modes/steps instead of leaving the flow. Verified on device.
• Un-ignored and de-flaked all four @Ignored e2e tests, including change phone number navigates back to EnterPhoneNumber step — the back path this change reworks, which had no coverage before.

⚠️ Breaking Changes

• EmailAuthScreen: mode and onNavigateToMode are now required.
• PhoneAuthScreen and MfaEnrollmentScreen: step, onNavigateToStep, onNavigateBack and flowState are now required.
• EmailAuthScreen no longer shows an error dialog of its own. This only affected a caller that installed LocalTopLevelDialogController itself; every caller still receives onError.

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Maintainer note: Fixes internal CPRN-415

[gemini-code-assist]

Code Review

This pull request refactors EmailAuthScreen, PhoneAuthScreen, and MfaEnrollmentScreen to always have their navigation steps driven externally by a host, removing the previous local state fallbacks. The demo activities are updated to host these screens on their own Navigation 3 back stacks, and several flaky UI tests are resolved by replacing arbitrary sleeps and state sampling with explicit waits on reported steps. Feedback was provided regarding PhoneAuthDestinations.kt, suggesting that the selectedCountry state should survive activity recreation using rememberSaveable to prevent potential mismatches between the country code and the entered phone number.