Implement CertificationStatusBadge widget

Create a private `_statusConfig(DateTime expiry)` method that returns a `_BadgeConfig` record (or small data class) containing `{Color background, Color foreground, String label, int daysRemaining}` — this separates the state-derivation logic from the layout code and makes it independently testable. Reference design tokens via the project's `AppColors` and `AppTextStyles` classes (e.g., `AppColors.certValid`, `AppColors.certExpiringSoon`, `AppColors.certExpired`). If these token names don't yet exist, add them to the design token file and use them consistently. For the HLF certification use case (primary driver of this widget per workshop notes), ensure the 'expiring_soon' state is visually prominent enough to motivate enrollment action.

Widget tests using flutter_test and `WidgetTester`. Test: (1) active state renders with active-color token and correct label, (2) expiring_soon state renders with warning-color token, (3) expired state renders with error-color token, (4) compact size renders without verbose label text, (5) full size renders with days-remaining text, (6) no hex color literals appear in the widget file (enforce via code review/grep). Use `find.byType(Container)` and inspect decoration to verify color token usage.