refactor(nav): split AppRoute by how a route arrives

[bmc08gt]

Stacked on #1417 — review that first.

AppRoute.Sheets held three unrelated populations: the four nav-bar tab homes, screens that are only ever pushed (ActivityHistory, TransactionDetails, Give), and actual modals. resolveRoutes wrapped every member in Main.Sheet, so the tab homes needed an asNavBarTab() carve-out to escape it and the pushed screens had no escape at all — a deeplink naming one landed in a modal instead of on the screen the rest of the app pushes.

Three groupings now say how a route arrives:

• Tabs — the four nav-bar homes (Scanner, Wallet, Tips, Menu). navBarButton() is exhaustive over the type, so a new tab home fails to compile until it is given a button rather than silently behaving as an ordinary push.
• Sheets — modals that host an inner backstack (TokenSelection, TipAmountEntry, ShareApp). That Main.Sheet packing is the only thing membership buys, so a modal needing no nested navigation implements com.getcode.navigation.Sheet directly instead — that marker, not this grouping, is what ModalBottomSheetSceneStrategy reads to present a route as a sheet.
• Main — ordinary pushes onto whatever stack the user is on.

resolveRoutes drops the carve-out and wraps on membership alone; leadsWithTab becomes routes.firstOrNull() is AppRoute.Tabs.

Behaviour

Unchanged in-app. Every entry to the three moved screens already goes through navigator.push (WalletScreen.kt:53, TokenInfoScreen.kt:195, CurrencyInfoExpansion.kt:123, ActivityHistoryScreen.kt:69), and AppRouter only ever feeds tab homes into navigateAll. The one change is the fix: a deeplink naming one of the three now lands on the pushed screen.

Route class names change, so a back stack saved by an older build will not restore across the update — inherent to any route rename. Maestro tags derive from simpleName, all of which are preserved, so the flows are untouched.

Three scanner destinations the new type made visible

Second commit. Once the tab homes are their own type, code that treats one as an ordinary push reads as a mistake rather than as a convention.

• The wallet's new-user tutorial dispatched "scan a tip card" as OpenScreen, which WalletScreen handles with navigator.push — the scanner arrived stacked on the wallet, and Back returned to the wallet instead of behaving like the nav bar. It now dispatches a SwitchTab event handled with replaceAll, the same call AppNavigationBar makes.
• PurchaseAccountScreen released to the scanner on account creation rather than to homeRoute, which every other onboarding exit uses.
• NotificationPermissionScreen.kt was a second copy of the permission screens left behind by the FlowHost migration, navigating to the scanner from both of its composables. Nothing calls them and AppRoute.Onboarding.NotificationPermission has no annotatedEntry to reach them by; the live path is OnboardingFlowScreen's NotificationPermissionStepContent, which releases to homeRoute. Deleted rather than repointed.