feat: add in-app onboarding guide for first-time users

ExampleApp/ExampleApp/ExampleAppApp.swift

@@ -42,9 +42,15 @@ struct ExampleAppApp: App {
   // Register app delegate for Firebase setup
   @UIApplicationDelegateAdaptor(AppDelegate.self) var delegate
 
+  @AppStorage("hasCompletedOnboarding") private var hasCompletedOnboarding = false
+
   var body: some Scene {
       WindowGroup {
-          ContentView()
+          if hasCompletedOnboarding {
+              ContentView()
+          } else {
+              OnboardingView()
+          }
       }
   }
 }

ExampleApp/ExampleApp/OnboardingView.swift

@@ -0,0 +1,156 @@
+//
+//  OnboardingView.swift
+//  ExampleApp
+//
+
+import SwiftUI
+
+struct OnboardingView: View {
+    @AppStorage("hasCompletedOnboarding") private var hasCompletedOnboarding = false
+    @State private var currentPage = 0
+
+    var body: some View {
+        ZStack {
+            Color.black.ignoresSafeArea()
+
+            VStack(spacing: 0) {
+                TabView(selection: $currentPage) {
+                    registerPage.tag(0)
+                    sessionPage.tag(1)
+                    capturePage.tag(2)
+                }
+                .tabViewStyle(.page(indexDisplayMode: .always))
+                .indexViewStyle(.page(backgroundDisplayMode: .always))
+
+                Spacer().frame(height: 32)
+
+                // Bottom button
+                Button(action: {
+                    if currentPage < 2 {
+                        withAnimation { currentPage += 1 }
+                    } else {
+                        hasCompletedOnboarding = true
+                    }
+                }) {
+                    Text(currentPage < 2 ? "Next" : "Get Started")
+                        .font(.headline)
+                        .foregroundColor(.white)
+                        .frame(maxWidth: .infinity)
+                        .padding(.vertical, 16)
+                        .background(.green)
+                        .cornerRadius(12)
+                }
+                .padding(.horizontal, 32)
+                .padding(.bottom, 48)
+
+                if currentPage < 2 {
+                    Button("Skip") {
+                        hasCompletedOnboarding = true
+                    }
+                    .font(.subheadline)
+                    .foregroundColor(.gray)
+                    .padding(.bottom, 24)
+                } else {
+                    Spacer().frame(height: 48)
+                }
+            }
+        }
+    }
+
+    // MARK: - Pages
+
+    private var registerPage: some View {
+        VStack(spacing: 24) {
+            Spacer()
+
+            stepBadge("Step 1")
+
+            Image(systemName: "shield.checkered")
+                .font(.system(size: 64))
+                .foregroundColor(.blue)
+
+            Text("Register your device")
+                .font(.title)
+                .fontWeight(.bold)
+                .foregroundColor(.white)
+
+            Text("Tap **Register Device** to link this device to SignedShot. This is a one-time setup that gives your device a unique identity for signing photos.")
+                .font(.body)
+                .foregroundColor(.gray)
+                .multilineTextAlignment(.center)
+                .padding(.horizontal, 32)
+
+            Spacer()
+            Spacer()
+        }
+    }
+
+    private var sessionPage: some View {
+        VStack(spacing: 24) {
+            Spacer()
+
+            stepBadge("Step 2")
+
+            Image(systemName: "play.circle.fill")
+                .font(.system(size: 64))
+                .foregroundColor(.green)
+
+            Text("Start a capture session")
+                .font(.title)
+                .fontWeight(.bold)
+                .foregroundColor(.white)
+
+            Text("Before each photo, tap **Start Session**. Sessions expire after 5 minutes as a security measure to prevent replay attacks.")
+                .font(.body)
+                .foregroundColor(.gray)
+                .multilineTextAlignment(.center)
+                .padding(.horizontal, 32)
+
+            Spacer()
+            Spacer()
+        }
+    }
+
+    private var capturePage: some View {
+        VStack(spacing: 24) {
+            Spacer()
+
+            stepBadge("Step 3")
+
+            Image(systemName: "camera.fill")
+                .font(.system(size: 64))
+                .foregroundColor(.orange)
+
+            Text("Capture & verify")
+                .font(.title)
+                .fontWeight(.bold)
+                .foregroundColor(.white)
+
+            Text("Take a photo with the capture button. Your photo and its cryptographic proof are saved to **Files → SignedShot**. Verify with the CLI or the API.")
+                .font(.body)
+                .foregroundColor(.gray)
+                .multilineTextAlignment(.center)
+                .padding(.horizontal, 32)
+
+            Spacer()
+            Spacer()
+        }
+    }
+
+    // MARK: - Helpers
+
+    private func stepBadge(_ text: String) -> some View {
+        Text(text)
+            .font(.caption)
+            .fontWeight(.semibold)
+            .foregroundColor(.white)
+            .padding(.horizontal, 12)
+            .padding(.vertical, 4)
+            .background(.white.opacity(0.15))
+            .cornerRadius(12)
+    }
+}
+
+#Preview {
+    OnboardingView()
+}