nav: fix blank back-gesture preview — let navigation drive predictive back

The per-screen Modifier.predictiveBack scaled the leaving screen with a
graphicsLayer, but NavHost only composes one back-stack entry at a time, so
nothing was drawn behind the shrinking screen — the gesture revealed the white
window background instead of the previous screen.

Drive predictive back through navigation-compose instead: it composes the
destination being revealed and seeks popExit with the gesture. popExit is now a
scaleOut + fade peek, so the leaving screen shrinks to reveal the real previous
screen held static. Drops the predictiveBack modifier from the destinations and
Settings (its inner section BackHandler is unchanged). The kit's predictiveBack
stays for overlay-style screens.

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

app/src/main/java/de/jeanlucmakiola/agendula/ui/navigation/AgendulaNavHost.kt

@@ -1,9 +1,10 @@
 package de.jeanlucmakiola.agendula.ui.navigation
 
-import de.jeanlucmakiola.floret.identity.predictiveBack
 import androidx.compose.animation.EnterTransition
 import androidx.compose.animation.ExitTransition
 import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.scaleOut
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.ui.Modifier
@@ -40,14 +41,13 @@ fun AgendulaNavHost(modifier: Modifier = Modifier) {
         modifier = modifier,
         // Only the screen on top moves; the background is always held static.
         // Forward is a plain fade-in over the held screen — calm, not a slide.
-        // Back is the system predictive-back peek: each destination applies
-        // Modifier.predictiveBack, so the leaving screen scales/rounds with the
-        // gesture and the held screen is revealed unmoved. NavHost itself does no
-        // pop animation (the screen owns the motion).
+        // Back is the predictive-back peek: navigation-compose composes the
+        // screen being revealed underneath and seeks popExit with the gesture, so
+        // the leaving screen scales down to reveal the (unmoved) previous screen.
         enterTransition = { fadeIn() },
         exitTransition = { ExitTransition.None },
         popEnterTransition = { EnterTransition.None },
-        popExitTransition = { ExitTransition.None },
+        popExitTransition = { scaleOut(targetScale = 0.9f) + fadeOut() },
     ) {
         composable(Dest.LISTS) {
             ListsScreen(
@@ -71,7 +71,6 @@ fun AgendulaNavHost(modifier: Modifier = Modifier) {
             LaunchedEffect(filter) { vm.bind(filter) }
 
             TaskListScreen(
-                modifier = Modifier.predictiveBack(onBack = { nav.popBackStack() }),
                 filter = filter,
                 viewModel = vm,
                 onOpenTask = { taskId -> nav.navigate(Dest.TaskDetail.build(taskId)) },
@@ -89,7 +88,6 @@ fun AgendulaNavHost(modifier: Modifier = Modifier) {
             LaunchedEffect(taskId) { vm.bind(taskId) }
 
             TaskDetailScreen(
-                modifier = Modifier.predictiveBack(onBack = { nav.popBackStack() }),
                 viewModel = vm,
                 onEdit = { nav.navigate(Dest.TaskEdit.buildEdit(taskId)) },
                 onDeleted = { nav.popBackStack() },
@@ -118,7 +116,6 @@ fun AgendulaNavHost(modifier: Modifier = Modifier) {
             }
 
             TaskEditScreen(
-                modifier = Modifier.predictiveBack(onBack = { nav.popBackStack() }),
                 viewModel = vm,
                 onSaved = { nav.popBackStack() },
                 onBack = { nav.popBackStack() },

app/src/main/java/de/jeanlucmakiola/agendula/ui/settings/SettingsScreen.kt

@@ -93,7 +93,6 @@ import de.jeanlucmakiola.floret.components.pastelize
 import de.jeanlucmakiola.floret.components.positionOf
 import de.jeanlucmakiola.floret.identity.collapseExit
 import de.jeanlucmakiola.floret.identity.expandEnter
-import de.jeanlucmakiola.floret.identity.predictiveBack
 import de.jeanlucmakiola.floret.reminders.ReminderOverride
 import de.jeanlucmakiola.agendula.ui.common.reminderLeadTimeLabel
 
@@ -128,9 +127,6 @@ fun SettingsScreen(
 
     Box(
         modifier = modifier
-            // At the hub, the back gesture previews the pop to the lists overview;
-            // in a section the BackHandler above takes it (section -> hub) instead.
-            .predictiveBack(onBack = onBack, enabled = section == null)
             .fillMaxSize()
             .background(MaterialTheme.colorScheme.surface),
     ) {