refactor: streamline main screen navigation and ViewModel injection

- Update `MainScreen` (Android) and `DesktopMainScreen` to manage their own `NavBackStack` initialization internally.
- Refactor `MainScreen` to obtain `UIViewModel` via Koin injection instead of receiving it as a parameter from `MainActivity`.
- Remove default bottom padding from `MeshtasticAppShell` and associated screen-level modifiers to allow for more flexible layout orchestration.
- Simplify `DesktopMainScreen` by moving backstack management inside the composable and cleaning up the navigation provider logic.
- Remove redundant lint suppressions in `Main.kt` following the simplification of the main screen composable structure.
- Clean up imports and normalize the usage of `MeshtasticNavDisplay` and `MeshtasticNavigationSuite` across platforms.

desktop/src/main/kotlin/org/meshtastic/desktop/Main.kt

@@ -321,7 +321,7 @@ fun main(args: Array<String>) = application(exitProcessOnExit = false) {
             // re-reads Locale.current and all stringResource() calls update.  Unlike key(), this
             // preserves remembered state (including the navigation backstack).
             CompositionLocalProvider(LocalAppLocale provides localePref) {
-                AppTheme(darkTheme = isDarkTheme) { DesktopMainScreen(backStack) }
+                AppTheme(darkTheme = isDarkTheme) { DesktopMainScreen(uiViewModel) }
             }
         }
     }

desktop/src/main/kotlin/org/meshtastic/desktop/ui/DesktopMainScreen.kt

@@ -18,41 +18,37 @@ package org.meshtastic.desktop.ui
 
 import androidx.compose.foundation.layout.fillMaxSize
 import androidx.compose.foundation.layout.padding
-import androidx.compose.material3.MaterialTheme
 import androidx.compose.material3.Surface
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.unit.dp
-import androidx.navigation3.runtime.NavBackStack
 import androidx.navigation3.runtime.NavKey
 import androidx.navigation3.runtime.entryProvider
-import org.koin.compose.viewmodel.koinViewModel
+import org.meshtastic.core.navigation.MeshtasticNavSavedStateConfig
+import org.meshtastic.core.navigation.NodesRoutes
 import org.meshtastic.core.ui.component.MeshtasticAppShell
 import org.meshtastic.core.ui.component.MeshtasticNavDisplay
+import org.meshtastic.core.ui.component.MeshtasticNavigationSuite
 import org.meshtastic.core.ui.viewmodel.UIViewModel
 import org.meshtastic.desktop.navigation.desktopNavGraph
 
 /**
- * Desktop main screen — Navigation 3 shell with adaptive navigation and shared [MeshtasticNavDisplay].
- *
- * Uses the same shared routes from `core:navigation` and the same `MeshtasticNavDisplay` + `entryProvider` pattern as
- * the Android app, proving the shared backstack architecture works across targets.
+ * Desktop main screen — uses shared navigation components.
  */
 @Composable
-@Suppress("LongMethod")
-fun DesktopMainScreen(backStack: NavBackStack<NavKey>, uiViewModel: UIViewModel = koinViewModel()) {
-    Surface(modifier = Modifier.fillMaxSize(), color = MaterialTheme.colorScheme.background) {
+fun DesktopMainScreen(uiViewModel: UIViewModel) {
+    val backStack = androidx.navigation3.runtime.rememberNavBackStack(MeshtasticNavSavedStateConfig, NodesRoutes.NodesGraph as NavKey)
+
+    Surface(modifier = Modifier.fillMaxSize()) {
         MeshtasticAppShell(
             backStack = backStack,
             uiViewModel = uiViewModel,
             hostModifier = Modifier.padding(bottom = 24.dp),
         ) {
-            org.meshtastic.core.ui.component.MeshtasticNavigationSuite(
-                backStack = backStack,
-                uiViewModel = uiViewModel,
-            ) {
-                val provider = entryProvider<NavKey> { desktopNavGraph(backStack, uiViewModel) }
-
+            MeshtasticNavigationSuite(backStack = backStack, uiViewModel = uiViewModel, modifier = Modifier.fillMaxSize()) {
+                val provider = entryProvider<NavKey> {
+                    desktopNavGraph(backStack, uiViewModel)
+                }
                 MeshtasticNavDisplay(backStack = backStack, entryProvider = provider, modifier = Modifier.fillMaxSize())
             }
         }