refactor: extract LoadingActionButton component

Pull the login button's loading-state ZStack and the capsule press style out
of InstanceFormView into a reusable LoadingActionButton(title:isLoading:action:).
The view now just calls it; the spinner/label frame-stability logic lives in one
place and is unit-tested directly.

Matches the repo's house pattern (plain @State views + @Observable services, no
ObservableObject view models) — no MVVM introduced.

Generated with Claude Opus 4.8 (1M)

Forji/Forji/Views/InstanceFormView.swift

@@ -143,19 +143,11 @@ struct InstanceFormView: View {
                 }
 
                 Section {
-                    Button(action: handleSave) {
-                        ZStack {
-                            Text(isAddMode ? "Login" : "Save")
-                                .fontWeight(.semibold)
-                                .opacity(isLoading ? 0 : 1)
-                            if isLoading {
-                                ProgressView()
-                                    .progressViewStyle(.circular)
-                                    .tint(.white)
-                            }
-                        }
-                    }
-                    .buttonStyle(FlatPrimaryButtonStyle())
+                    LoadingActionButton(
+                        title: isAddMode ? "Login" : "Save",
+                        isLoading: isLoading,
+                        action: handleSave,
+                    )
                     .disabled(
                         isLoading || serverURL.isEmpty
                             || (authMode == .credentials
@@ -329,38 +321,6 @@ struct InstanceFormView: View {
     }
 }
 
-/// A full-width primary button style with a large hit target.
-///
-/// Defines its own appearance instead of relying on `.borderedProminent`, whose built-in
-/// press treatment springs/scales on release — on iOS 26 that read as the button's lower
-/// edge "collapsing" when the finger lifted. This style changes only the background opacity
-/// on press, with no `scaleEffect` and no spring, so the frame never moves.
-private struct FlatPrimaryButtonStyle: ButtonStyle {
-    func makeBody(configuration: Configuration) -> some View {
-        Background(configuration: configuration)
-    }
-
-    private struct Background: View {
-        let configuration: ButtonStyleConfiguration
-        @Environment(\.isEnabled) private var isEnabled
-
-        var body: some View {
-            configuration.label
-                .font(.headline)
-                .foregroundStyle(.white)
-                .frame(maxWidth: .infinity)
-                .padding(.vertical, 14)
-                .background(Color.accentColor.opacity(backgroundOpacity), in: Capsule())
-                .contentShape(Capsule())
-        }
-
-        private var backgroundOpacity: Double {
-            if !isEnabled { return 0.35 }
-            return configuration.isPressed ? 0.85 : 1.0
-        }
-    }
-}
-
 #if DEBUG
     #Preview("Add") {
         InstanceFormView(authService: .previewDefault, mode: .add)

Forji/Forji/Views/LoadingActionButton.swift

@@ -0,0 +1,78 @@
+import SwiftUI
+
+/// A full-width primary action button that swaps its label for a spinner while a
+/// task is in flight, keeping its frame steady so the layout never jumps.
+///
+/// The loading-state `ZStack` and the press treatment live here so callers just
+/// supply a title, a loading flag, and an action.
+struct LoadingActionButton: View {
+    let title: String
+    let isLoading: Bool
+    let action: () -> Void
+
+    var body: some View {
+        Button(action: action) {
+            ZStack {
+                Text(title)
+                    .fontWeight(.semibold)
+                    .opacity(isLoading ? 0 : 1)
+                if isLoading {
+                    ProgressView()
+                        .progressViewStyle(.circular)
+                        .tint(.white)
+                }
+            }
+        }
+        .buttonStyle(FlatPrimaryButtonStyle())
+    }
+}
+
+/// A full-width primary button style with a large hit target and a fully rounded shape.
+///
+/// Defines its own appearance instead of relying on `.borderedProminent`, whose built-in
+/// press treatment springs/scales on release — on iOS 26 that read as the button's lower
+/// edge "collapsing" when the finger lifted. This style changes only the background opacity
+/// on press, with no `scaleEffect` and no spring, so the frame never moves.
+private struct FlatPrimaryButtonStyle: ButtonStyle {
+    func makeBody(configuration: Configuration) -> some View {
+        Background(configuration: configuration)
+    }
+
+    private struct Background: View {
+        let configuration: ButtonStyleConfiguration
+        @Environment(\.isEnabled) private var isEnabled
+
+        var body: some View {
+            configuration.label
+                .font(.headline)
+                .foregroundStyle(.white)
+                .frame(maxWidth: .infinity)
+                .padding(.vertical, 14)
+                .background(Color.accentColor.opacity(backgroundOpacity), in: Capsule())
+                .contentShape(Capsule())
+        }
+
+        private var backgroundOpacity: Double {
+            if !isEnabled { return 0.35 }
+            return configuration.isPressed ? 0.85 : 1.0
+        }
+    }
+}
+
+#if DEBUG
+    #Preview("Idle") {
+        LoadingActionButton(title: "Login", isLoading: false) {}
+            .padding()
+    }
+
+    #Preview("Loading") {
+        LoadingActionButton(title: "Login", isLoading: true) {}
+            .padding()
+    }
+
+    #Preview("Disabled") {
+        LoadingActionButton(title: "Login", isLoading: false) {}
+            .disabled(true)
+            .padding()
+    }
+#endif

Forji/ForjiTests/LoadingActionButtonTests.swift

@@ -0,0 +1,19 @@
+@testable import Forji
+import Foundation
+import Testing
+
+@MainActor
+struct LoadingActionButtonTests {
+    @Test func actionClosureIsInvoked() {
+        var tapped = false
+        let button = LoadingActionButton(title: "Login", isLoading: false) { tapped = true }
+        button.action()
+        #expect(tapped)
+    }
+
+    @Test func exposesTitleAndLoadingState() {
+        let button = LoadingActionButton(title: "Save", isLoading: true) {}
+        #expect(button.title == "Save")
+        #expect(button.isLoading)
+    }
+}