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.

core/ui/src/commonMain/kotlin/org/meshtastic/core/ui/component/MeshtasticAppShell.kt

@@ -16,11 +16,9 @@
  */
 package org.meshtastic.core.ui.component
 
-import androidx.compose.foundation.layout.padding
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.ui.Modifier
-import androidx.compose.ui.unit.dp
 import androidx.navigation3.runtime.NavBackStack
 import androidx.navigation3.runtime.NavKey
 import org.meshtastic.core.navigation.NodeDetailRoutes
@@ -36,7 +34,7 @@ import org.meshtastic.core.ui.viewmodel.UIViewModel
 fun MeshtasticAppShell(
     backStack: NavBackStack<NavKey>,
     uiViewModel: UIViewModel,
-    hostModifier: Modifier = Modifier.padding(bottom = 16.dp),
+    hostModifier: Modifier = Modifier,
     content: @Composable () -> Unit,
 ) {
     LaunchedEffect(uiViewModel) {