fix: avoid startup crash when saved appdata path is unavailable

WheelWizard.Test/Features/Settings/PathManagerTests.cs

@@ -0,0 +1,27 @@
+using WheelWizard.Services;
+
+namespace WheelWizard.Test.Features.Settings;
+
+[Collection("SettingsFeature")]
+public class PathManagerTests
+{
+    [Fact]
+    public void TrySetWheelWizardAppdataPath_ReturnsFalse_WhenTargetPathIsUnavailable()
+    {
+        if (!OperatingSystem.IsWindows())
+            return;
+
+        var unavailablePath = GetUnavailableWindowsPath();
+        var result = PathManager.TrySetWheelWizardAppdataPath(unavailablePath, out var errorMessage, out _);
+
+        Assert.False(result);
+        Assert.False(string.IsNullOrWhiteSpace(errorMessage));
+    }
+
+    private static string GetUnavailableWindowsPath()
+    {
+        var used = DriveInfo.GetDrives().Select(d => char.ToUpperInvariant(d.Name[0])).ToHashSet();
+        var drive = "ZYXWVUTSRQPONMLKJIHGFEDCBA".FirstOrDefault(letter => !used.Contains(letter), 'Z');
+        return $@"{drive}:\WheelWizardTests\{Guid.NewGuid():N}";
+    }
+}

WheelWizard/Services/PathManager.cs

@@ -95,14 +95,35 @@ public static bool IsUsingCustomWheelWizardAppdataPath
                 return null;
 
             var normalized = FileHelper.NormalizePath(storedPath);
-            return FileHelper.PathsEqual(normalized, DefaultWheelWizardAppdataPath) ? null : normalized;
+            if (FileHelper.PathsEqual(normalized, DefaultWheelWizardAppdataPath))
+                return null;
+
+            // If a previously selected custom location is no longer available (for example,
+            // when an external drive letter changes), fall back to the default path.
+            if (!TryEnsureWheelWizardAppdataPathAccessible(normalized))
+                return null;
+
+            return normalized;
         }
         catch
         {
             return null;
         }
     }
 
+    private static bool TryEnsureWheelWizardAppdataPathAccessible(string normalizedPath)
+    {
+        try
+        {
+            FileHelper.EnsureDirectory(normalizedPath);
+            return true;
+        }
+        catch
+        {
+            return false;
+        }
+    }
+
     private static string? LoadPersistedWheelWizardAppdataOverride()
     {
 #if WINDOWS