fix: parse fee rate with the invariant culture

On Italian/German systems "1.5" was read as 15 sat/vB. The Send form,
the Donate form and the CLI now share CoinAmount.TryParseFeeRate
(invariant culture, '.' or ',' as decimal separator, range (0, 1000]).
Donate and the CLI report an invalid rate instead of silently using 1.

src/App/ViewModels/MainWindowViewModel.Donate.cs

@@ -44,9 +44,11 @@ public partial class MainWindowViewModel
                 DonatePreview = Loc.Tr("msg.amount.invalid");
                 return;
             }
-            if (!decimal.TryParse(SendFeeRate, System.Globalization.NumberStyles.Any,
-                    System.Globalization.CultureInfo.InvariantCulture, out var feeRate) || feeRate <= 0)
-                feeRate = 1m;
+            if (!CoinAmount.TryParseFeeRate(SendFeeRate, out var feeRate))
+            {
+                DonatePreview = Loc.Tr("msg.feerate.invalid");
+                return;
+            }
 
             _pendingDonate = new TransactionFactory(_account).Build(
                 _doc.Cache.Utxos, _lastTransactions, destination, amount, feeRate,

src/App/ViewModels/MainWindowViewModel.Send.cs

@@ -74,7 +74,7 @@ public partial class MainWindowViewModel
                 SendPreview = Loc.Tr("msg.amount.invalid");
                 return;
             }
-            if (!decimal.TryParse(SendFeeRate, out var feeRate) || feeRate <= 0)
+            if (!CoinAmount.TryParseFeeRate(SendFeeRate, out var feeRate))
             {
                 SendPreview = Loc.Tr("msg.feerate.invalid");
                 return;

src/Cli/Program.cs

@@ -217,7 +217,9 @@ static async Task<int> Send(string[] o)
     long amount = 0;
     if (!sendAll && !CoinAmount.TryParseCoins(Opt(o, "--amount") ?? "", out amount))
         throw new WalletSpendException("--amount missing or invalid (or use --all).");
-    var feeRate = decimal.TryParse(Opt(o, "--feerate"), out var fr) ? fr : 1m;
+    var feeRate = 1m;
+    if (Opt(o, "--feerate") is { } feeText && !CoinAmount.TryParseFeeRate(feeText, out feeRate))
+        throw new WalletSpendException($"--feerate invalid (expected sat/vB in (0, {CoinAmount.MaxFeeRate}]).");
 
     // Source transactions of the UTXOs: needed for amounts and signing.
     await using var client = await Connect(o, account.Profile);

src/Core/Wallet/CoinAmount.cs

@@ -80,4 +80,25 @@ public static class CoinAmount
         }
         return true;
     }
+
+    /// <summary>Upper bound accepted for a fee rate, in sat/vB (guards against typos).</summary>
+    public const decimal MaxFeeRate = 1000m;
+
+    /// <summary>
+    /// Parses a fee rate in sat/vB, independent of the system culture. Accepts a decimal point or
+    /// comma; rejects thousands separators, exponents, signs, zero and values above
+    /// <see cref="MaxFeeRate"/>.
+    /// </summary>
+    public static bool TryParseFeeRate(string? text, out decimal satPerVbyte)
+    {
+        satPerVbyte = 0;
+        if (text is null)
+            return false;
+        text = text.Trim().Replace(',', '.');
+        if (!decimal.TryParse(text, NumberStyles.AllowDecimalPoint, CultureInfo.InvariantCulture, out var rate)
+            || rate <= 0 || rate > MaxFeeRate)
+            return false;
+        satPerVbyte = rate;
+        return true;
+    }
 }