feat(01-05): implement TranslationSource singleton + EN/FR resx files

- TranslationSource singleton with INotifyPropertyChanged indexer binding
- PropertyChanged fires with string.Empty on culture switch (signals all bindings refresh)
- Missing key returns [key] placeholder (prevents null in WPF bindings)
- Strings.resx with 27 Phase 1 UI string keys (EN)
- Strings.fr.resx with same 27 keys stubbed with EN text (FR completeness Phase 5)
- Strings.Designer.cs ResourceManager for dotnet build compatibility
- SharepointToolbox.csproj updated with EmbeddedResource metadata

SharepointToolbox/Localization/TranslationSource.cs

@@ -0,0 +1,38 @@
+using System.ComponentModel;
+using System.Globalization;
+using System.Resources;
+
+namespace SharepointToolbox.Localization;
+
+/// <summary>
+/// Singleton INotifyPropertyChanged string lookup enabling runtime culture switching without restart.
+/// WPF bindings use: Source={x:Static loc:TranslationSource.Instance}, Path=[key]
+/// On CurrentCulture change, fires PropertyChanged with empty string to signal all properties changed,
+/// which causes WPF to re-evaluate all bindings to this source.
+/// </summary>
+public class TranslationSource : INotifyPropertyChanged
+{
+    public static readonly TranslationSource Instance = new();
+
+    private ResourceManager _resourceManager = Strings.ResourceManager;
+    private CultureInfo _currentCulture = CultureInfo.CurrentUICulture;
+
+    private TranslationSource() { }
+
+    public string this[string key] =>
+        _resourceManager.GetString(key, _currentCulture) ?? $"[{key}]";
+
+    public CultureInfo CurrentCulture
+    {
+        get => _currentCulture;
+        set
+        {
+            if (Equals(_currentCulture, value)) return;
+            _currentCulture = value;
+            Thread.CurrentThread.CurrentUICulture = value;
+            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(string.Empty));
+        }
+    }
+
+    public event PropertyChangedEventHandler? PropertyChanged;
+}