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/Core/Models/AppSettings.cs

@@ -0,0 +1,7 @@
+namespace SharepointToolbox.Core.Models;
+
+public class AppSettings
+{
+    public string DataFolder { get; set; } = string.Empty;
+    public string Lang { get; set; } = "en";
+}