android: fix navigation backstack handling

As seen previously in #66

android/src/main/kotlin/dev/msfjarvis/claw/android/ui/LobstersApp.kt

@@ -122,7 +122,7 @@ fun LobstersApp(
       ) {
         NavHost(
           navController,
-          startDestination = Destinations.Hottest.getRoute(),
+          startDestination = Destinations.startDestination.getRoute(),
           modifier = Modifier.padding(top = 8.dp),
         ) {
           composable(Destinations.Hottest.getRoute()) {

android/src/main/kotlin/dev/msfjarvis/claw/android/ui/decorations/ClawNavigationBar.kt

@@ -19,6 +19,7 @@ import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.painter.Painter
 import androidx.navigation.NavController
 import dev.msfjarvis.claw.android.ui.AnimationDuration
+import dev.msfjarvis.claw.android.ui.navigation.Destinations
 
 @Composable
 fun ClawNavigationBar(
@@ -53,7 +54,11 @@ fun ClawNavigationBar(
           selected = selectedIndex == index,
           onClick = {
             selectedIndex = index
-            navController.navigate(navItem.route)
+            if (navController.currentDestination?.route == navItem.route) return@NavigationBarItem
+            navController.popBackStack(navController.graph.startDestinationRoute!!, false)
+            if (navItem.route != Destinations.startDestination.getRoute()) {
+              navController.navigate(navItem.route)
+            }
           }
         )
       }

android/src/main/kotlin/dev/msfjarvis/claw/android/ui/navigation/Destinations.kt

@@ -12,4 +12,9 @@ sealed class Destinations(internal val route: String) {
   object Comments : Destinations("comments/%s") {
     fun getRoute(postId: String) = route.format(postId)
   }
+
+  companion object {
+    val startDestination
+      get() = Hottest
+  }
 }