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.

gradle/libs.versions.toml

@@ -74,6 +74,7 @@ material = { module = "com.google.android.material:material", version.ref = "mat
 androidx-compose-bom = { module = "androidx.compose:compose-bom", version.ref = "composeBom" }
 androidx-compose-ui = { module = "androidx.compose.ui:ui" }
 androidx-compose-ui-graphics = { module = "androidx.compose.ui:ui-graphics" }
+androidx-compose-foundation = { module = "androidx.compose.foundation:foundation" }
 androidx-compose-ui-tooling = { module = "androidx.compose.ui:ui-tooling" }
 androidx-compose-ui-tooling-preview = { module = "androidx.compose.ui:ui-tooling-preview" }
 androidx-compose-ui-test-junit4 = { module = "androidx.compose.ui:ui-test-junit4" }
@@ -127,6 +128,7 @@ androidx-test-espresso-core = { module = "androidx.test.espresso:espresso-core",
 compose = [
     "androidx-compose-ui",
     "androidx-compose-ui-graphics",
+    "androidx-compose-foundation",
     "androidx-compose-ui-tooling-preview",
     "androidx-compose-material3",
 ]