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.

build-logic/convention/src/main/kotlin/com/example/architecture/convention/ComposeConventionPlugin.kt

@@ -26,6 +26,9 @@ class ComposeConventionPlugin : Plugin<Project> {
         }
 
         dependencies {
+            // `implementation` (not api): every Compose consumer applies this convention itself, so
+            // Compose must NOT leak transitively — that keeps the UI-agnostic presentation module
+            // (which depends on core:presentation) free of Compose.
             val bom = platform(libs.findLibrary("androidx-compose-bom").get())
             add("implementation", bom)
             add("androidTestImplementation", bom)