M8f: instant navigation, no transitions (D-108)

NavHost enter, exit, pop and size transitions are None, so pushed screens,
tab switches, editors and the session gate change at once instead of the
default 700 ms cross-fade. NavTransitionsTest asserts the values.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

android/app/src/main/kotlin/se/hagstedt/kvitt/nav/KvittApp.kt

@@ -149,7 +149,16 @@ private fun AppGraph(viewModel: AppViewModel, session: SessionState, updateCheck
 
     // Result snackbars are shown in the scaffold of the screen returned to (Home, S-5, S-8, S-10, S-12, S-16), which
     // each host [snackbar] (D-071).
-    NavHost(nav, startDestination = start) {
+    // D-108: every destination change is instant (NavTransitions).
+    NavHost(
+        nav,
+        startDestination = start,
+        enterTransition = NavTransitions.enter,
+        exitTransition = NavTransitions.exit,
+        popEnterTransition = NavTransitions.enter,
+        popExitTransition = NavTransitions.exit,
+        sizeTransform = NavTransitions.size,
+    ) {
         onboarding(viewModel, shell)
         topLevel(session, shell)
         accountsAndExpenses(viewModel, shell)

android/app/src/main/kotlin/se/hagstedt/kvitt/nav/NavTransitions.kt

@@ -0,0 +1,27 @@
+package se.hagstedt.kvitt.nav
+
+import androidx.compose.animation.AnimatedContentTransitionScope
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import androidx.compose.animation.SizeTransform
+import androidx.navigation.NavBackStackEntry
+
+/**
+ * Destination changes are instant (D-108): no enter, exit, pop or size animation for pushed screens, tab switches,
+ * editors or the session gate (`NavHost` otherwise cross-fades every change over 700 ms). Motion inside a screen
+ * (ripples, expanding sections, the FAB, sheets) belongs to the components and is not affected.
+ *
+ * The `NavHost` lambdas only delegate to [enterTransition], [exitTransition] and [sizeTransform], which the unit
+ * test asserts (the transition scope is sealed and cannot be built outside a composition).
+ */
+internal object NavTransitions {
+    fun enterTransition(): EnterTransition = EnterTransition.None
+
+    fun exitTransition(): ExitTransition = ExitTransition.None
+
+    fun sizeTransform(): SizeTransform? = null
+
+    val enter: AnimatedContentTransitionScope<NavBackStackEntry>.() -> EnterTransition = { enterTransition() }
+    val exit: AnimatedContentTransitionScope<NavBackStackEntry>.() -> ExitTransition = { exitTransition() }
+    val size: AnimatedContentTransitionScope<NavBackStackEntry>.() -> SizeTransform? = { sizeTransform() }
+}

android/app/src/test/kotlin/se/hagstedt/kvitt/nav/NavTransitionsTest.kt

@@ -0,0 +1,25 @@
+package se.hagstedt.kvitt.nav
+
+import androidx.compose.animation.EnterTransition
+import androidx.compose.animation.ExitTransition
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertSame
+import org.junit.Test
+
+/** D-108: the app's `NavHost` changes destination without any animation. */
+class NavTransitionsTest {
+    @Test
+    fun enterIsNone() {
+        assertSame(EnterTransition.None, NavTransitions.enterTransition())
+    }
+
+    @Test
+    fun exitIsNone() {
+        assertSame(ExitTransition.None, NavTransitions.exitTransition())
+    }
+
+    @Test
+    fun noSizeTransform() {
+        assertNull(NavTransitions.sizeTransform())
+    }
+}