Added different startup screen and ability to open and save project into files

Examples/Nodify.Calculator/StartupWindow.xaml

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+<Window x:Class="Nodify.Calculator.StartupWindow"
+        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
+        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
+        Title="API Visual Executor"
+        Width="500"
+        Height="380"
+        WindowStartupLocation="CenterScreen"
+        ResizeMode="NoResize"
+        Background="#1E1E1E"
+        Foreground="White">
+    <Grid>
+        <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center" Width="360">
+            <TextBlock Text="API Visual Executor"
+                       FontSize="26" FontWeight="Bold"
+                       Foreground="#6366f1"
+                       HorizontalAlignment="Center"
+                       Margin="0 0 0 8" />
+            <TextBlock Text="Create or open a project to get started"
+                       FontSize="13" Foreground="Gray"
+                       HorizontalAlignment="Center"
+                       Margin="0 0 0 30" />
+
+            <Button Content="➕  New Project"
+                    Height="48" FontSize="16"
+                    Margin="0 0 0 14"
+                    Background="#6366f1" Foreground="White"
+                    BorderThickness="0" Cursor="Hand"
+                    Click="NewProject_Click" />
+
+            <Button Content="📂  Open Project"
+                    Height="48" FontSize="16"
+                    Margin="0 0 0 14"
+                    Background="#3E3E42" Foreground="White"
+                    BorderThickness="1" BorderBrush="#555"
+                    Cursor="Hand"
+                    Click="OpenProject_Click" />
+        </StackPanel>
+    </Grid>
+</Window>