fix(app): preserve tab slot state and remove duplicate bottom insets

- Wrap tab crossfade slots in SaveableStateHolder to preserve slot-local state across tab switches

- Remove redundant navigationBarsPadding modifier on ABoxNavigationBar to prevent duplicated bottom spacing

android/app/src/main/kotlin/com/flagship/abox/manager/ui/shell/MainAppShell.kt

@@ -6,7 +6,6 @@ import androidx.compose.foundation.layout.Box
 import androidx.compose.foundation.layout.Column
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.fillMaxWidth
-import androidx.compose.foundation.layout.navigationBarsPadding
 import androidx.compose.foundation.layout.padding
 import androidx.compose.foundation.layout.statusBarsPadding
 import androidx.compose.material3.ExperimentalMaterial3Api
@@ -19,6 +18,7 @@ import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
 import androidx.compose.runtime.saveable.rememberSaveable
+import androidx.compose.runtime.saveable.rememberSaveableStateHolder
 import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
@@ -56,6 +56,7 @@ fun MainAppShell(
 ) {
     var activeTab by rememberSaveable { mutableStateOf(initialTab) }
     var showAccountSheet by remember { mutableStateOf(false) }
+    val tabStateHolder = rememberSaveableStateHolder()
 
     Scaffold(
         modifier = modifier.fillMaxSize(),
@@ -77,9 +78,7 @@ fun MainAppShell(
             }
         },
         bottomBar = {
-            ABoxNavigationBar(
-                modifier = Modifier.navigationBarsPadding(),
-            ) {
+            ABoxNavigationBar {
                 MainTab.values().forEach { tab ->
                     val isSelected = (tab == activeTab)
                     ABoxNavigationBarItem(
@@ -111,10 +110,12 @@ fun MainAppShell(
                 targetState = activeTab,
                 label = "MainTabCrossfade",
             ) { tab ->
-                when (tab) {
-                    MainTab.DEVICES -> devicesSlot()
-                    MainTab.SCENES -> scenesSlot()
-                    MainTab.ACTIVITY -> activitySlot()
+                tabStateHolder.SaveableStateProvider(tab) {
+                    when (tab) {
+                        MainTab.DEVICES -> devicesSlot()
+                        MainTab.SCENES -> scenesSlot()
+                        MainTab.ACTIVITY -> activitySlot()
+                    }
                 }
             }
         }