feat(01-03): SettingsRepository and SettingsService with write-then-replace

- AppSettings model: DataFolder + Lang with camelCase JSON serialization
- SettingsRepository: SemaphoreSlim write lock + write-then-replace (tmp→validate→move)
- SettingsService: GetSettings/SetLanguage/SetDataFolder; SetLanguage validates en/fr only
- All 8 SettingsServiceTests pass; all 18 Unit tests pass

SharepointToolbox.Tests/Services/SettingsServiceTests.cs

@@ -1,7 +1,123 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Persistence;
+using SharepointToolbox.Services;
+
 namespace SharepointToolbox.Tests.Services;
 
-public class SettingsServiceTests
+[Trait("Category", "Unit")]
+public class SettingsServiceTests : IDisposable
 {
-    [Fact(Skip = "Wave 0 stub — implemented in plan 01-03")]
-    public void SaveAndLoad_RoundTrips_Settings() { }
+    private readonly string _tempFile;
+
+    public SettingsServiceTests()
+    {
+        _tempFile = Path.GetTempFileName();
+        File.Delete(_tempFile);
+    }
+
+    public void Dispose()
+    {
+        if (File.Exists(_tempFile)) File.Delete(_tempFile);
+        if (File.Exists(_tempFile + ".tmp")) File.Delete(_tempFile + ".tmp");
+    }
+
+    private SettingsRepository CreateRepository() => new(_tempFile);
+    private SettingsService CreateService() => new(CreateRepository());
+
+    [Fact]
+    public async Task LoadAsync_MissingFile_ReturnsDefaultSettings()
+    {
+        var repo = CreateRepository();
+
+        var settings = await repo.LoadAsync();
+
+        Assert.Equal(string.Empty, settings.DataFolder);
+        Assert.Equal("en", settings.Lang);
+    }
+
+    [Fact]
+    public async Task SaveAndLoad_RoundTrips_DataFolderAndLang()
+    {
+        var repo = CreateRepository();
+        var original = new AppSettings { DataFolder = @"C:\Exports", Lang = "fr" };
+
+        await repo.SaveAsync(original);
+        var loaded = await repo.LoadAsync();
+
+        Assert.Equal(@"C:\Exports", loaded.DataFolder);
+        Assert.Equal("fr", loaded.Lang);
+    }
+
+    [Fact]
+    public async Task SaveAsync_SerializedJson_UsesDataFolderAndLangKeys()
+    {
+        var repo = CreateRepository();
+        await repo.SaveAsync(new AppSettings { DataFolder = @"C:\Exports", Lang = "fr" });
+
+        var json = await File.ReadAllTextAsync(_tempFile);
+        using var doc = JsonDocument.Parse(json);
+
+        Assert.True(doc.RootElement.TryGetProperty("dataFolder", out _),
+            "JSON must contain 'dataFolder' key (camelCase for schema compatibility)");
+        Assert.True(doc.RootElement.TryGetProperty("lang", out _),
+            "JSON must contain 'lang' key (camelCase for schema compatibility)");
+    }
+
+    [Fact]
+    public async Task SaveAsync_UsesTmpFileThenMove()
+    {
+        var repo = CreateRepository();
+
+        // The .tmp file should not exist after a successful save
+        await repo.SaveAsync(new AppSettings { DataFolder = @"C:\Test", Lang = "en" });
+
+        Assert.False(File.Exists(_tempFile + ".tmp"),
+            "Temp file should have been moved/deleted after successful save");
+        Assert.True(File.Exists(_tempFile), "Settings file must exist after save");
+    }
+
+    [Fact]
+    public async Task SetLanguageAsync_PersistsLang()
+    {
+        var service = CreateService();
+
+        await service.SetLanguageAsync("fr");
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal("fr", settings.Lang);
+    }
+
+    [Fact]
+    public async Task SetDataFolderAsync_PersistsPath()
+    {
+        var service = CreateService();
+
+        await service.SetDataFolderAsync(@"C:\Exports");
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal(@"C:\Exports", settings.DataFolder);
+    }
+
+    [Fact]
+    public async Task SetDataFolderAsync_EmptyString_IsAllowed()
+    {
+        var service = CreateService();
+        await service.SetDataFolderAsync(@"C:\Exports");
+
+        await service.SetDataFolderAsync(string.Empty);
+
+        var settings = await service.GetSettingsAsync();
+        Assert.Equal(string.Empty, settings.DataFolder);
+    }
+
+    [Fact]
+    public async Task SetLanguageAsync_InvalidCode_ThrowsArgumentException()
+    {
+        var service = CreateService();
+
+        await Assert.ThrowsAsync<ArgumentException>(() => service.SetLanguageAsync("de"));
+    }
 }