feat(android): add design system components matching web theme

Implement 10 reusable Jetpack Compose UI components:
- ShieldButton: 4 variants (primary/secondary/ghost/danger), 3 sizes, loading state, icon support
- ShieldCard: gradient background matching web .gradient-card, click handling, header/footer slots
- ShieldTextField: validation, password toggle, error/helper text, focus styling
- ShieldBadge: 5 variants (default/success/warning/error/info), pill shape, icon support
- ShieldModal: ModalBottomSheet + AlertDialog, swipe-to-dismiss
- ShieldToast: Snackbar-based with 4 variants, auto-dismiss, action buttons
- ShieldAvatar: Coil async image loading, initials fallback, online status indicator
- ShieldProgressBar: linear progress with percentage, 5 color variants
- ShieldEmptyState: icon, title, description, action button
- ShieldSkeleton: shimmer animation with infinite transition

All components use theme tokens (no hardcoded colors) and support light/dark modes.

Add Coil dependency for avatar image loading.
Add ComponentShowcase preview with light/dark mode support.
Add instrumented Compose UI tests for all components.

android/ShieldAI/gradle/libs.versions.toml

@@ -9,6 +9,7 @@ activityCompose = "1.8.0"
 navigationCompose = "2.7.7"
 kotlin = "2.2.10"
 composeBom = "2025.12.00"
+coilCompose = "2.7.0"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }
@@ -27,6 +28,7 @@ androidx-compose-ui-test-manifest = { group = "androidx.compose.ui", name = "ui-
 androidx-compose-ui-test-junit4 = { group = "androidx.compose.ui", name = "ui-test-junit4" }
 androidx-compose-material3 = { group = "androidx.compose.material3", name = "material3" }
 androidx-compose-material3-adaptive-navigation-suite = { group = "androidx.compose.material3", name = "material3-adaptive-navigation-suite" }
+coil-compose = { group = "io.coil-kt", name = "coil-compose", version.ref = "coilCompose" }
 
 [plugins]
 android-application = { id = "com.android.application", version.ref = "agp" }