identity: add the shared predictive-back peek (Modifier.predictiveBack)

The standard Android back-gesture preview — the surface scales to ~90%, rounds
its corners and shifts toward the swiped edge as the gesture is dragged, then
commits or springs back. Plus rememberReduceMotion() to honour the system
"remove animations" setting. Drawn from Calendula's CalendarTransitions so both
apps preview back the same way; adds the activity-compose dependency.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

identity/build.gradle.kts

@@ -36,4 +36,6 @@ dependencies {
     implementation(libs.androidx.ui)
     implementation(libs.androidx.foundation)
     implementation(libs.androidx.material3)
+    // activity-compose for PredictiveBackHandler + BackEventCompat (the back-gesture peek).
+    implementation(libs.androidx.activity.compose)
 }