feat(buy): allow the buy flow to be pushed onto a host stack

Flipcash/Core/Navigation/AppRouter+Destination.swift

@@ -29,6 +29,9 @@ extension AppRouter {
         /// Wallet's Recent section. `transactionHistory` is the per-token slice.
         case activity
         case give(PublicKey)
+        /// Pushes the buy flow (`BuyAmountScreen`) onto the current stack instead
+        /// of presenting it as a sheet — the new-UI currency-info "Convert"/"Get".
+        case buyCurrency(PublicKey)
         /// Skips the picker step in the withdraw flow and lands the user on
         /// `WithdrawIntroScreen` with the currency pre-selected. Pushed from
         /// USDF Currency Info on the Wallet sheet.
@@ -74,7 +77,7 @@ extension AppRouter {
             switch self {
             case .currencyInfo, .currencyInfoForDeposit, .discoverCurrencies,
                  .currencyCreationSummary, .currencyCreationWizard,
-                 .transactionHistory, .activity, .give, .withdrawCurrency,
+                 .transactionHistory, .activity, .give, .buyCurrency, .withdrawCurrency,
                  .usdcDepositEducation, .usdcDepositAddress:
                 return .balance
             case .settingsMyAccount, .settingsAdvancedFeatures, .settingsAdvancedBetaFeatures,
@@ -102,6 +105,7 @@ extension AppRouter {
             case .transactionHistory:           "transactionHistory"
             case .activity:                     "activity"
             case .give:                         "give"
+            case .buyCurrency:                  "buyCurrency"
             case .withdrawCurrency:             "withdrawCurrency"
             case .usdcDepositEducation:         "usdcDepositEducation"
             case .usdcDepositAddress:           "usdcDepositAddress"
@@ -133,6 +137,7 @@ extension AppRouter {
                  .currencyInfoForDeposit(let mint),
                  .transactionHistory(let mint),
                  .give(let mint),
+                 .buyCurrency(let mint),
                  .withdrawCurrency(let mint):
                 return mint.base58
             case .tipConversation(let conversationID),

Flipcash/Core/Navigation/AppRouter+DestinationView.swift

@@ -59,6 +59,9 @@ struct DestinationView: View {
             GiveScreen(mint: mint)
                 .id(mint)
 
+        case .buyCurrency(let mint):
+            BuyAmountScreen(mint: mint)
+
         // MARK: - Settings flow
 
         case .settingsMyAccount:

Flipcash/Core/Navigation/AppRouter+NestedSheet.swift

@@ -86,23 +86,17 @@ private struct BuySheetRoot: View {
 
     let mint: PublicKey
 
-    @Environment(SessionContainer.self) private var sessionContainer
     @Environment(AppRouter.self) private var router
 
     var body: some View {
         @Bindable var router = router
         NavigationStack(path: $router[.buy]) {
-            BuyAmountScreen(
-                mint: mint,
-                currencyName: sessionContainer.session.balance(for: mint)?.name ?? "this currency",
-                session: sessionContainer.session,
-                ratesController: sessionContainer.ratesController
-            )
-            .id(mint)
-            .environment(\.dismissParentContainer, { router.dismissSheet() })
-            // Registers the shared destinations, including the Add Money
-            // deposit steps, so a buy-shortfall deposit pushes onto this stack.
-            .appRouterDestinations()
+            BuyAmountScreen(mint: mint)
+                .environment(\.dismissParentContainer, { router.dismissSheet() })
+                .environment(\.presentedAsSheetRoot, true)
+                // Registers the shared destinations, including the Add Money
+                // deposit steps, so a buy-shortfall deposit pushes onto this stack.
+                .appRouterDestinations()
         }
     }
 }

Flipcash/Core/Screens/Main/Buy/BuyAmountScreen.swift

@@ -9,13 +9,41 @@ import SwiftUI
 import FlipcashCore
 import FlipcashUI
 
+/// Amount entry for buying a currency. The caller provides the surrounding
+/// `NavigationStack` — the buy sheet wraps it, the pushed `.buyCurrency`
+/// destination uses the active one.
+///
+/// Thin environment-reading wrapper that hands the DI containers to
+/// ``BuyAmountScreenContent``, whose `init` seeds the `@State` view model
+/// synchronously. `.id(mint)` rebuilds the content (and its view model) when the
+/// mint changes.
 struct BuyAmountScreen: View {
 
+    let mint: PublicKey
+
+    @Environment(SessionContainer.self) private var sessionContainer
+
+    var body: some View {
+        BuyAmountScreenContent(
+            mint: mint,
+            currencyName: sessionContainer.session.balance(for: mint)?.name ?? "this currency",
+            session: sessionContainer.session,
+            ratesController: sessionContainer.ratesController
+        )
+        .id(mint)
+    }
+}
+
+private struct BuyAmountScreenContent: View {
+
     @State private var viewModel: BuyAmountViewModel
     @State private var isShowingCurrencySelection: Bool = false
 
     @Environment(AppRouter.self) private var router
     @Environment(RatesController.self) private var ratesController
+    /// True when this is the root of a presented sheet; false when pushed onto a
+    /// host stack (new-UI Convert/Get) where the system back arrow replaces Close.
+    @Environment(\.presentedAsSheetRoot) private var presentedAsSheetRoot
 
     init(mint: PublicKey, currencyName: String, session: Session, ratesController: RatesController) {
         self._viewModel = State(initialValue: BuyAmountViewModel(
@@ -56,7 +84,9 @@ struct BuyAmountScreen: View {
         .navigationTitle(viewModel.screenTitle)
         .toolbarTitleDisplayMode(.inline)
         .toolbar {
-            if !isDismissBlocked {
+            // Only the sheet-root presentation gets a Close button; a pushed
+            // instance relies on the system back arrow.
+            if presentedAsSheetRoot && !isDismissBlocked {
                 ToolbarItem(placement: .topBarTrailing) {
                     CloseButton(action: router.dismissSheet)
                 }