feat(core:design-system): AppTheme + reusable composables (REDI-81)

- AppTheme wraps Material3 (color scheme, typography, shapes); all previews use it.
- Slot-API AppCard (header + content slots, optional click); AppScaffold.
- LoadingIndicator, ErrorState (optional retry), Coil-backed NetworkImage.
- Modifier.shimmerEffect() animated placeholder (Modifier extension, not @Composable).
- Add androidx-compose-foundation to the version catalog + compose bundle.

core/design-system/build.gradle.kts

@@ -6,3 +6,9 @@ plugins {
 android {
     namespace = "com.example.architecture.core.design.system"
 }
+
+dependencies {
+    // Coil is internal to NetworkImage; no Coil types leak into public signatures.
+    implementation(libs.coil.compose)
+    implementation(libs.coil.network.okhttp)
+}