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/app/build.gradle.kts

@@ -50,6 +50,7 @@ dependencies {
     implementation(libs.androidx.compose.ui.tooling.preview)
     implementation(libs.androidx.compose.material3)
     implementation(libs.androidx.compose.material3.adaptive.navigation.suite)
+    implementation(libs.coil.compose)
     testImplementation(libs.junit)
     androidTestImplementation(libs.androidx.junit)
     androidTestImplementation(libs.androidx.espresso.core)