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/Infrastructure/Persistence/SettingsRepository.cs

@@ -0,0 +1,74 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Infrastructure.Persistence;
+
+public class SettingsRepository
+{
+    private readonly string _filePath;
+    private readonly SemaphoreSlim _writeLock = new(1, 1);
+
+    public SettingsRepository(string filePath)
+    {
+        _filePath = filePath;
+    }
+
+    public async Task<AppSettings> LoadAsync()
+    {
+        if (!File.Exists(_filePath))
+            return new AppSettings();
+
+        string json;
+        try
+        {
+            json = await File.ReadAllTextAsync(_filePath, Encoding.UTF8);
+        }
+        catch (IOException ex)
+        {
+            throw new InvalidDataException($"Failed to read settings file: {_filePath}", ex);
+        }
+
+        try
+        {
+            var settings = JsonSerializer.Deserialize<AppSettings>(json,
+                new JsonSerializerOptions { PropertyNameCaseInsensitive = true });
+            return settings ?? new AppSettings();
+        }
+        catch (JsonException ex)
+        {
+            throw new InvalidDataException($"Settings file contains invalid JSON: {_filePath}", ex);
+        }
+    }
+
+    public async Task SaveAsync(AppSettings settings)
+    {
+        await _writeLock.WaitAsync();
+        try
+        {
+            var json = JsonSerializer.Serialize(settings,
+                new JsonSerializerOptions
+                {
+                    WriteIndented = true,
+                    PropertyNamingPolicy = JsonNamingPolicy.CamelCase
+                });
+
+            var tmpPath = _filePath + ".tmp";
+            var dir = Path.GetDirectoryName(_filePath);
+            if (!string.IsNullOrEmpty(dir))
+                Directory.CreateDirectory(dir);
+
+            await File.WriteAllTextAsync(tmpPath, json, Encoding.UTF8);
+
+            // Validate round-trip before replacing
+            JsonDocument.Parse(await File.ReadAllTextAsync(tmpPath, Encoding.UTF8)).Dispose();
+
+            File.Move(tmpPath, _filePath, overwrite: true);
+        }
+        finally
+        {
+            _writeLock.Release();
+        }
+    }
+}