UX touch-friendly: add Evaluate button; increase min sizes/spacing; make variables/functions tap-to-insert via InsertVariable/InsertFunction commands.

src/AdvancedCalculator/ViewModels/MainViewModel.cs

@@ -41,6 +41,37 @@ public partial class MainViewModel : ViewModelBase
         IsFunctionsPanelOpen = !IsFunctionsPanelOpen;
     }
 
+    // Insert helpers for touch: appends tokens to the input box
+    [RelayCommand]
+    private void InsertVariable(string? variableName)
+    {
+        if (string.IsNullOrWhiteSpace(variableName))
+            return;
+        InsertToken(variableName);
+    }
+
+    [RelayCommand]
+    private void InsertFunction(string? functionName)
+    {
+        if (string.IsNullOrWhiteSpace(functionName))
+            return;
+        InsertToken(functionName + "()");
+    }
+
+    private void InsertToken(string token)
+    {
+        if (string.IsNullOrEmpty(InputText))
+        {
+            InputText = token;
+            return;
+        }
+
+        if (!char.IsWhiteSpace(InputText[^1]))
+            InputText += " ";
+
+        InputText += token;
+    }
+
     [RelayCommand(AllowConcurrentExecutions = false)]
     private async Task Submit()
     {