refactor: Update MainActivity for edge-to-edge display support

- Implemented modern edge-to-edge display handling for Android 15+ in MainActivity.kt.
- Removed deprecated SystemUiMode.edgeToEdge configuration from main.dart, as edge-to-edge is now managed natively.
- Ensured backward compatibility by maintaining the previous edge-to-edge display settings for lower Android versions.

android/app/src/main/kotlin/app/cogwheel/conduit/MainActivity.kt

@@ -10,10 +10,15 @@ class MainActivity : FlutterActivity() {
     private lateinit var backgroundStreamingHandler: BackgroundStreamingHandler
     
     override fun onCreate(savedInstanceState: Bundle?) {
-        // Ensure content draws behind system bars (backwards compatible helper)
-        WindowCompat.setDecorFitsSystemWindows(window, false)
-        
         super.onCreate(savedInstanceState)
+        
+        // Modern edge-to-edge implementation for Android 15+
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.VANILLA_ICE_CREAM) {
+            window.decorView.setOnApplyWindowInsetsListener(null)
+        }
+        
+        // Enable edge-to-edge display (backwards compatible)
+        WindowCompat.setDecorFitsSystemWindows(window, false)
     }
     
     override fun configureFlutterEngine(flutterEngine: FlutterEngine) {