fix(build): revert ui-tooling-preview to implementation scope

The previous batch fix tried to move ui-tooling-preview to
debugImplementation per a reviewer suggestion, but @Preview is used
in MainActivity.kt which lives in the main source set, so the
annotation class must be available at release-build compile time.

Moving @Preview composables to a debug-only source set would let the
dep stay debug-scoped - that is a Plan 02+ refactor, not foundation
work.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -92,7 +92,7 @@ dependencies {
     implementation(platform(libs.androidx.compose.bom))
     implementation(libs.androidx.ui)
     implementation(libs.androidx.ui.graphics)
-    debugImplementation(libs.androidx.ui.tooling.preview)
+    implementation(libs.androidx.ui.tooling.preview)
     implementation(libs.androidx.material3)
 
     implementation(libs.hilt.android)