basic GUI structure implemented, along with -c or --cli flags to run in a CLI rather than launch the GUI.

src/App.java

@@ -0,0 +1,58 @@
+import javax.swing.JFrame;
+import javax.swing.JPanel;
+import javax.swing.UIManager;
+
+import java.awt.BorderLayout;
+import java.awt.Dimension;
+
+/**
+ * Runner class for the Sudoku App, managing the GUI and coordinating internal logic.
+ */
+public class App extends JFrame {
+    private Settings settings;
+
+    public App() {
+        super("Sudoku");
+        settings = new Settings();
+        initSetup();
+    }
+
+    /**
+     * Basic JFrame setup for the GUI.
+     */
+    private void initSetup() {
+        // TODO: These are just some default values, these should use the values from Settings.
+        setDefaultCloseOperation(EXIT_ON_CLOSE);
+        setLayout(new BorderLayout());
+        setPreferredSize(new Dimension(750, 550));
+        setSize(getPreferredSize());
+        setResizable(false);
+        setLocationRelativeTo(null);
+
+        try {
+            UIManager.setLookAndFeel(UIManager.getCrossPlatformLookAndFeelClassName());
+        } catch (Exception e) {
+            System.out.println("Error with cross platform look/feel in frameSetup().");
+        }
+    }
+
+    private JPanel createApp() {
+        // TODO: Create the root JPanel holding all other GUI components.
+
+        return null;
+    }
+
+    public static void cli() {
+        Nav nav = new Nav(new Settings());
+        SudokuChecker check = new SudokuChecker(nav.getLoadedGrid());
+    }
+
+    public static void main(String[] args) {
+        if(args.length == 0)
+            new App().setVisible(true);
+        else if(args[0].equals("-c") || args[0].equals("--cli"))
+            cli();
+        else
+            new App().setVisible(true);
+    }
+}