Add isPremium badge support to UIOnboardingFeature

[FelixLisczyk]

Summary

This PR adds an optional premium badge overlay to feature rows in the onboarding screen.

New properties on UIOnboardingFeature

• isPremium: Bool (default: false) — when true, renders a small capsule badge over the bottom-trailing corner of the feature icon
• premiumBadgeTitle: String (default: "Pro") — the text shown inside the badge, always rendered uppercase

Both properties are additive with default values, so existing code requires no changes.

Badge rendering

The badge is implemented directly in UIOnboardingCell as a UIView overlay using Auto Layout. It is anchored to the bottom-trailing corner of the feature glyph and correctly handles cell reuse (removed and re-evaluated on every configure(feature:) call).

Demo apps updated

All three demo apps (UIOnboarding Demo, UIOnboarding Demo SPM, UIOnboarding SwiftUI) have been updated to showcase the badge on the #teamarmee feature using the app's camouflage tint color.

Usage

UIOnboardingFeature(
    icon: UIImage(named: "feature-3")!,
    iconTint: .systemBlue,
    isPremium: true,
    premiumBadgeTitle: "Pro", // rendered as "PRO"
    title: "Cloud Sync",
    description: "Access your data on all your devices."
)

Screenshots