feat(onboarding): collect display name in place of phone verification

[bmc08gt]

What

Ports the Android onboarding change (code-android-app#1183) to iOS: replaces the phone-verification step in onboarding with a mandatory display-name step. Onboarding is now ordered:

create account → access key → name → permissions (push)

Why

We want onboarding to capture a display name (used for tip cards, chat, etc.). The iOS phone step existed solely to power phone-number Send, which was removed in #546 — so the step was vestigial. Replacing it with display-name collection is the natural follow-through now that Tips is the default.

How

• Drop phone verification from onboarding. EnterPhoneScreen / ConfirmPhoneScreen / PhoneVerificationViewModel are untouched — they're still used by the post-login Send sheet and the Onramp flow. Only onboarding's entry point changes: the .phoneVerification / .confirmPhoneNumberCode OnboardingPath cases and the enablePhoneNumberSend gate are gone.
• Add the name step. New OnboardingNameViewModel + OnboardingNameScreen collect the name pre-login, reusing DisplayNameValidator and SetDisplayName against the in-flight owner keypair. The tip-card ProfileNameScreen can't be reused as-is (it's session-scoped: SessionContainer / AppRouter), so this mirrors how the onboarding phone step wrapped its own view model.
• Mandatory step. The navigationDestination hides the back button, so a new account always has a name before reaching the app.
• Behavioral note. Because the name is captured during onboarding, the tip-card setup's name prompt is now redundant for new accounts (they land in tips already named). Existing users who onboarded before this change still get the tip-setup name path — that screen is unchanged.

Testing

• App builds clean; build-for-testing compiles all test targets.
• New OnboardingNameViewModelTests (validation-state coverage) — 3 passing.
• Updated the create-account UI smoke helpers (enterDisplayNameIfNeeded replaces allowPhoneVerificationIfNeeded on the onboarding path) across CreateAccount, CurrencySelection, DiscoverCurrencies, Give, and createFreshAccount.