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/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())
+    }
+}