Adding OpenHaystack Mobile app

Co-Authored-By: Lukas Burg <lukas.burg@hemalu.de>

openhaystack-mobile/lib/splashscreen.dart

@@ -0,0 +1,28 @@
+import 'package:flutter/material.dart';
+
+class Splashscreen extends StatelessWidget {
+
+  /// Display a fullscreen splashscreen to cover loading times.
+  const Splashscreen({ Key? key }) : super(key: key);
+
+  @override
+  Widget build(BuildContext context) {
+    Size screenSize = MediaQuery.of(context).size;
+    Orientation orientation = MediaQuery.of(context).orientation;
+
+    var maxScreen = orientation == Orientation.portrait ? screenSize.width : screenSize.height;
+    var maxSize = maxScreen * 0.4;
+
+    return Scaffold(
+      body: Center(
+        child: Container(
+          constraints: BoxConstraints(maxWidth: maxSize, maxHeight: maxSize),
+          // TODO: Update app icon accordingly (https://docs.flutter.dev/development/ui/assets-and-images#platform-assets)
+          child: const Image(
+            width: 1800,
+            image: AssetImage('assets/OpenHaystackIcon.png')),
+        ),
+      ),
+    );
+  }
+}