refactor: migrate AmountInputViewModel to @observable

Replaces the legacy ObservableObject/@StateObject/@ObservedObject pattern with @observable + @State, aligning with the project's stated SwiftUI direction. No behavior change: there are no two-way bindings to the view model, so no @bindable is needed. Touches the view model, NumberPadTextField, and the 8 amount-entry call sites.

Author: CypherPoet

Bitkit/Components/NumberPadTextField.swift

@@ -3,7 +3,7 @@ import SwiftUI
 /// NumberPadTextField - Amount view to be used with number pad
 struct NumberPadTextField: View {
     @EnvironmentObject var currency: CurrencyViewModel
-    @ObservedObject var viewModel: AmountInputViewModel
+    var viewModel: AmountInputViewModel
 
     var showConversion: Bool = true
     var showEditButton: Bool = false

Bitkit/ViewModels/AmountInputViewModel.swift

@@ -1,11 +1,12 @@
 import Foundation
 import SwiftUI
 
+@Observable
 @MainActor
-class AmountInputViewModel: ObservableObject {
-    @Published var amountSats: UInt64 = 0
-    @Published var displayText: String = ""
-    @Published var errorKey: String?
+final class AmountInputViewModel {
+    var amountSats: UInt64 = 0
+    var displayText: String = ""
+    var errorKey: String?
 
     /// Optional per-screen cap (e.g. the max sendable balance in the send flow).
     /// When set, input is additionally blocked above this value, on top of `maxAmount`.

Bitkit/Views/Transfer/FundManualAmountView.swift

@@ -8,7 +8,7 @@ struct FundManualAmountView: View {
 
     let lnPeer: LnPeer
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
     @State private var didAttemptPeerConnection = false
 
     var amountSats: UInt64 {

Bitkit/Views/Transfer/SpendingAdvancedView.swift

@@ -10,7 +10,7 @@ struct SpendingAdvancedView: View {
     @EnvironmentObject var transfer: TransferViewModel
     @Environment(\.dismiss) var dismiss
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
     @State private var feeEstimate: UInt64?
     @State private var isLoading = false
     @State private var feeEstimateTask: Task<Void, Never>?

Bitkit/Views/Transfer/SpendingAmount.swift

@@ -10,7 +10,7 @@ struct SpendingAmount: View {
     @EnvironmentObject var transfer: TransferViewModel
     @EnvironmentObject var wallet: WalletViewModel
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
     @State private var isLoading = false
     @State private var availableAmount: UInt64?
     @State private var maxTransferAmount: UInt64?

Bitkit/Views/Wallets/LnurlWithdraw/LnurlWithdrawAmount.swift

@@ -6,7 +6,7 @@ struct LnurlWithdrawAmount: View {
     @EnvironmentObject var wallet: WalletViewModel
     let onContinue: () -> Void
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
 
     var minAmount: Int {
         Int(max(1, app.lnurlWithdrawData!.minWithdrawableSat))

Bitkit/Views/Wallets/Receive/ReceiveCjitAmount.swift

@@ -9,7 +9,7 @@ struct ReceiveCjitAmount: View {
 
     @Binding var navigationPath: [ReceiveRoute]
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
 
     var minimumAmount: UInt64 {
         blocktank.minCjitSats ?? 0

Bitkit/Views/Wallets/Receive/ReceiveEdit.swift

@@ -12,7 +12,7 @@ struct ReceiveEdit: View {
 
     @Binding var navigationPath: [ReceiveRoute]
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
     @State private var note = ""
     @State private var isAmountInputFocused: Bool = false
     @FocusState private var isNoteEditorFocused: Bool

Bitkit/Views/Wallets/Send/LnurlPayAmount.swift

@@ -7,7 +7,7 @@ struct LnurlPayAmount: View {
 
     @Binding var navigationPath: [SendRoute]
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
 
     var maxAmount: UInt64 {
         // TODO: subtract fee

Bitkit/Views/Wallets/Send/SendAmountView.swift

@@ -8,7 +8,7 @@ struct SendAmountView: View {
 
     @Binding var navigationPath: [SendRoute]
 
-    @StateObject private var amountViewModel = AmountInputViewModel()
+    @State private var amountViewModel = AmountInputViewModel()
     @State private var maxSendableAmount: UInt64?
     @State private var routingFee: UInt64 = 0