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.

feature/intro/src/androidMain/kotlin/org/meshtastic/feature/intro/AppIntroductionScreen.kt

@@ -24,6 +24,7 @@ import com.google.accompanist.permissions.ExperimentalPermissionsApi
 import com.google.accompanist.permissions.PermissionState
 import com.google.accompanist.permissions.rememberMultiplePermissionsState
 import com.google.accompanist.permissions.rememberPermissionState
+import org.meshtastic.core.ui.component.MeshtasticNavDisplay
 
 /**
  * Main application introduction screen. This Composable hosts the navigation flow and hoists the permission states.
@@ -56,7 +57,7 @@ fun AppIntroductionScreen(onDone: () -> Unit, viewModel: IntroViewModel) {
 
     val backStack = rememberNavBackStack(Welcome)
 
-    org.meshtastic.core.ui.component.MeshtasticNavDisplay(
+    MeshtasticNavDisplay(
         backStack = backStack,
         entryProvider =
         introNavGraph(