fix: parse stock amounts invariantly

Modules/StocksModule.cs

@@ -116,7 +116,7 @@ public async Task StockSellAsync(string target, string amountStr = "all")
         decimal? sharesToSell = null;
         if (amountStr.ToLower() != "all")
         {
-            if (!decimal.TryParse(amountStr, out decimal parsed) || parsed <= 0)
+            if (!StockInputParser.TryParsePositiveAmount(amountStr, out decimal parsed))
             {
                 await ReplyAsync("Invalid amount. Use a positive number or `all`.");
                 return;
@@ -168,7 +168,7 @@ public async Task StockTransferAsync(string target, IUser targetUser, string amo
         decimal? sharesToTransfer = null;
         if (amountStr.ToLower() != "all")
         {
-            if (!decimal.TryParse(amountStr, out decimal parsed) || parsed <= 0)
+            if (!StockInputParser.TryParsePositiveAmount(amountStr, out decimal parsed))
             {
                 await ReplyAsync("Invalid amount. Use a positive number or `all`.");
                 return;

Morpheus.Tests/StockInputParserTests.cs

@@ -0,0 +1,39 @@
+using System.Globalization;
+using Morpheus.Utilities;
+
+namespace Morpheus.Tests;
+
+public class StockInputParserTests
+{
+    [Fact]
+    public void TryParsePositiveAmount_UsesInvariantDecimalSeparator()
+    {
+        CultureInfo originalCulture = CultureInfo.CurrentCulture;
+        try
+        {
+            CultureInfo culture = (CultureInfo)CultureInfo.InvariantCulture.Clone();
+            culture.NumberFormat.NumberDecimalSeparator = ",";
+            culture.NumberFormat.NumberGroupSeparator = ".";
+            CultureInfo.CurrentCulture = culture;
+
+            bool parsed = StockInputParser.TryParsePositiveAmount("12.50", out decimal amount);
+
+            Assert.True(parsed);
+            Assert.Equal(12.50m, amount);
+        }
+        finally
+        {
+            CultureInfo.CurrentCulture = originalCulture;
+        }
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("0")]
+    [InlineData("-1")]
+    public void TryParsePositiveAmount_RejectsMissingAndNonPositiveValues(string? input)
+    {
+        Assert.False(StockInputParser.TryParsePositiveAmount(input, out _));
+    }
+}

Utilities/StockInputParser.cs

@@ -0,0 +1,11 @@
+using System.Globalization;
+
+namespace Morpheus.Utilities;
+
+internal static class StockInputParser
+{
+    public static bool TryParsePositiveAmount(string? input, out decimal amount)
+    {
+        return decimal.TryParse(input, NumberStyles.Number, CultureInfo.InvariantCulture, out amount) && amount > 0;
+    }
+}