Fix text truncation in onboarding views

Add .fixedSize(horizontal: false, vertical: true) to prevent
multiline text from being clipped on smaller screens.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Shared/Onboarding/views/OnboardingWelcome.swift

@@ -48,6 +48,7 @@ struct OnboardingWelcome: View {
                     .font(.headline.weight(.medium))
                     .foregroundColor(.white.opacity(0.9))
                     .multilineTextAlignment(.center)
+                    .fixedSize(horizontal: false, vertical: true)
                     .padding(.horizontal, 40)
 
                 Spacer()