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

[felippemsc]

Summary

• Add 3-step onboarding walkthrough shown on first launch
  • Step 1: Register your device (one-time setup)
  • Step 2: Start a capture session (5-min security window)
  • Step 3: Capture & verify (files saved to Files → SignedShot)
• Uses @AppStorage to only show once, skippable via "Skip" button
• New OnboardingView.swift file, minor change to ExampleAppApp.swift entry point

Test plan

• Delete app and reinstall — onboarding should appear
• Swipe through all 3 pages and tap "Get Started"
• Relaunch app — onboarding should NOT appear again
• Delete app, reinstall, tap "Skip" on first page — should go straight to camera
• Verify text is readable and not truncated on smaller devices (iPhone SE)