nav: spring slide/fade screen transitions (Calendula feel)

Drive the NavHost with the motion scheme's fastSpatialSpec: forward fades in
over a held, static background; back (and the predictive-back drag) slides the
top screen out to the right, revealing the screen beneath unmoved — the same
overlay feel as Calendula, instead of the default navigation-compose transition.

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

app/src/main/java/de/jeanlucmakiola/floret/ui/navigation/FloretNavHost.kt

@@ -1,5 +1,10 @@
 package de.jeanlucmakiola.floret.ui.navigation
 
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.fadeIn
+import androidx.compose.animation.fadeOut
+import androidx.compose.animation.slideOutHorizontally
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.ui.Modifier
@@ -28,11 +33,20 @@ import de.jeanlucmakiola.floret.ui.tasklist.TaskListViewModel
 @Composable
 fun FloretNavHost(modifier: Modifier = Modifier) {
     val nav = rememberNavController()
+    val slide = rememberNavSlideSpec()
 
     NavHost(
         navController = nav,
         startDestination = Dest.LISTS,
         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 (and the predictive-back drag, which drives popExit) is the snappy
+        // slide-out to the right, revealing the held screen unmoved.
+        enterTransition = { fadeIn() },
+        exitTransition = { ExitTransition.None },
+        popEnterTransition = { EnterTransition.None },
+        popExitTransition = { slideOutHorizontally(slide) { it } + fadeOut() },
     ) {
         composable(Dest.LISTS) {
             ListsScreen(

app/src/main/java/de/jeanlucmakiola/floret/ui/navigation/FloretTransitions.kt

@@ -0,0 +1,21 @@
+package de.jeanlucmakiola.floret.ui.navigation
+
+import androidx.compose.animation.core.FiniteAnimationSpec
+import androidx.compose.material3.ExperimentalMaterial3ExpressiveApi
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.unit.IntOffset
+
+/**
+ * The horizontal slide spec for screen-to-screen navigation: the *fast*
+ * spring-physics spec from the active motion scheme — snappy with a subtle
+ * springy settle, rather than a fixed easing curve. Same choice as Calendula's
+ * calendar slide, so the two apps move the same way.
+ *
+ * Read in composable scope so the non-composable `NavHost` transition lambdas
+ * can capture it.
+ */
+@OptIn(ExperimentalMaterial3ExpressiveApi::class)
+@Composable
+fun rememberNavSlideSpec(): FiniteAnimationSpec<IntOffset> =
+    MaterialTheme.motionScheme.fastSpatialSpec()