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/Strings.Designer.cs

@@ -0,0 +1,60 @@
+//------------------------------------------------------------------------------
+// <auto-generated>
+//     This code was generated by a tool.
+//     Runtime Version:4.0.30319.42000
+//
+//     Changes to this file may cause incorrect behavior and will be lost if
+//     the code is regenerated.
+// </auto-generated>
+//------------------------------------------------------------------------------
+
+namespace SharepointToolbox.Localization {
+    using System;
+    using System.Reflection;
+
+    /// <summary>
+    ///   A strongly-typed resource class, for looking up localized strings, etc.
+    ///   Auto-generated designer file for Strings.resx — do not edit manually.
+    /// </summary>
+    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("System.Resources.Tools.StronglyTypedResourceBuilder", "17.0.0.0")]
+    [global::System.Diagnostics.DebuggerNonUserCodeAttribute()]
+    [global::System.Runtime.CompilerServices.CompilerGeneratedAttribute()]
+    internal class Strings {
+
+        private static global::System.Resources.ResourceManager resourceMan;
+
+        private static global::System.Globalization.CultureInfo resourceCulture;
+
+        [global::System.Diagnostics.CodeAnalysis.SuppressMessageAttribute("Microsoft.Performance", "CA1811:AvoidUncalledPrivateCode")]
+        internal Strings() {
+        }
+
+        /// <summary>
+        ///   Returns the cached ResourceManager instance used by this class.
+        /// </summary>
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Advanced)]
+        internal static global::System.Resources.ResourceManager ResourceManager {
+            get {
+                if (object.ReferenceEquals(resourceMan, null)) {
+                    global::System.Resources.ResourceManager temp = new global::System.Resources.ResourceManager("SharepointToolbox.Localization.Strings", typeof(Strings).Assembly);
+                    resourceMan = temp;
+                }
+                return resourceMan;
+            }
+        }
+
+        /// <summary>
+        ///   Overrides the current thread's CurrentUICulture property for all
+        ///   resource lookups using this strongly typed resource class.
+        /// </summary>
+        [global::System.ComponentModel.EditorBrowsableAttribute(global::System.ComponentModel.EditorBrowsableState.Advanced)]
+        internal static global::System.Globalization.CultureInfo Culture {
+            get {
+                return resourceCulture;
+            }
+            set {
+                resourceCulture = value;
+            }
+        }
+    }
+}