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/Services/SettingsService.cs

@@ -0,0 +1,39 @@
+using SharepointToolbox.Core.Models;
+using SharepointToolbox.Infrastructure.Persistence;
+
+namespace SharepointToolbox.Services;
+
+public class SettingsService
+{
+    private readonly SettingsRepository _repository;
+
+    private static readonly HashSet<string> SupportedLanguages = new(StringComparer.OrdinalIgnoreCase)
+    {
+        "en", "fr"
+    };
+
+    public SettingsService(SettingsRepository repository)
+    {
+        _repository = repository;
+    }
+
+    public Task<AppSettings> GetSettingsAsync()
+        => _repository.LoadAsync();
+
+    public async Task SetLanguageAsync(string cultureCode)
+    {
+        if (!SupportedLanguages.Contains(cultureCode))
+            throw new ArgumentException($"Unsupported language code '{cultureCode}'. Supported: en, fr.", nameof(cultureCode));
+
+        var settings = await _repository.LoadAsync();
+        settings.Lang = cultureCode;
+        await _repository.SaveAsync(settings);
+    }
+
+    public async Task SetDataFolderAsync(string path)
+    {
+        var settings = await _repository.LoadAsync();
+        settings.DataFolder = path;
+        await _repository.SaveAsync(settings);
+    }
+}