feat(onboarding): unify login into a single OnboardingFlow with linear FlowHost API

[bmc08gt]

Replace the fragmented login routing (LoginRouter, standalone screens, and scattered verification logic in MainRoot) with a single OnboardingFlowScreen backed by two FlowHost phases: Account and Permissions.

FlowHost API changes:

• Add linear flow overload with steps, resumeAt, and completedResult parameters for ordered step-by-step flows
• Add FlowNavigator.proceed() to advance through the step list, exit with completedResult at the end, or delegate to onProceed for custom behavior
• Rename the existing overload as the non-linear variant for flows that manage their own navigation via navigateTo/exitWithResult
• Extract shared logic into FlowHostImpl; support re-seeding when initialStack changes before user navigation (async flag settling)

Onboarding routing:

• All AuthState.Registered cases now route to AppRoute.OnboardingFlow with a ResumePoint (Login, AccessKey, AccessKeyThenPurchase, or PostAccessKey) — MainRoot no longer routes directly to Verification
• PostAccessKeyRedirect checks UserProfile.verifiedPhoneNumber to skip verification when phone is already linked
• Seed restore (LoggedIn) skips verification and goes straight to permissions — existing users encounter phone verification in-app via the send flow
• Permissions phase uses the linear FlowHost with resumeAt to skip already-granted permissions

Login module restructuring:

• Delete LoginRouter, AccessKeyScreen, SeedInputScreen (standalone wrappers) — all step content is now composed inline by OnboardingFlowScreen via the entryProvider
• Move ViewModels to internal package, screen content to internal/screens
• Add OnboardingStep sealed interface and OnboardingResult for flow step/result modeling