fix(auth): handle system back press in phone auth flow

[just1and0]

Summary

• Adds BackHandler to PhoneAuthScreen so the system back button is handled consistently for both default and custom phone content
• The base handler delegates to onCancel(), which navigates to the method picker or prevents premature activity finish in phone-only flows
• The default UI adds a step-specific override on the verification code screen: system back returns to phone number entry instead of cancelling
• Custom phoneContent slots inherit the base behavior and can override with their own BackHandler

Fixes #2058

Test plan

• Launch phone auth as the only provider — verify system back does not finish the activity
• Launch phone auth with multiple providers — verify system back returns to method picker
• On verification code screen — verify system back returns to phone number entry
• With custom phoneContent slot — verify consumer BackHandler overrides the default

[gemini-code-assist]

Code Review

This pull request integrates BackHandler into the PhoneAuthScreen to manage back navigation during the phone authentication flow. The review feedback recommends disabling back navigation while authentication or verification requests are in progress to prevent race conditions and ensure state consistency.

[gemini-code-assist]

To prevent race conditions or inconsistent states, the back button should be disabled while an authentication operation is in progress (isLoading is true). If the user is allowed to navigate back or cancel while a network request is active, the background task might still succeed or fail later, leading to unexpected state transitions (e.g., suddenly logging in the user or showing an error dialog after they have already navigated away).

Suggested change

	BackHandler { onCancel() }
	BackHandler(enabled = !isLoading) { onCancel() }

[gemini-code-assist]

Similarly, the back handler on the verification code screen should be disabled while a verification request is in progress (state.isLoading is true). This prevents the user from returning to the phone number entry screen while the code is actively being verified, which could otherwise lead to a race condition if the verification completes successfully in the background.

Suggested change

	BackHandler { state.onChangeNumberClick() }
	BackHandler(enabled = !state.isLoading) { state.onChangeNumberClick() }

[just1and0]

Closing — no user interest on the original issue (no thumbs up), and confirmed by team that this isn't worth pursuing.