feat(01-02): add SharePointPaginationHelper, ExecuteQueryRetryHelper, LogPanelSink

- SharePointPaginationHelper: async iterator with ListItemCollectionPosition loop (bypasses 5k limit); RowLimit=2000; [EnumeratorCancellation] for correct WithCancellation support
- ExecuteQueryRetryHelper: exponential backoff on 429/503/throttle; surfaces retry events via IProgress<OperationProgress>; max 5 retries
- LogPanelSink: custom Serilog ILogEventSink writing color-coded entries to RichTextBox via Dispatcher.InvokeAsync for thread safety

SharepointToolbox/Infrastructure/Logging/LogPanelSink.cs

@@ -0,0 +1,49 @@
+using Serilog.Core;
+using Serilog.Events;
+using System.Windows;
+using System.Windows.Documents;
+using System.Windows.Media;
+using System.Windows.Controls;
+
+namespace SharepointToolbox.Infrastructure.Logging;
+
+/// <summary>
+/// Custom Serilog sink that writes timestamped, color-coded entries to a WPF RichTextBox.
+/// Format: HH:mm:ss [LEVEL] Message — green=info/success, orange=warning, red=error.
+/// All writes dispatch to the UI thread via Application.Current.Dispatcher.
+/// </summary>
+public class LogPanelSink : ILogEventSink
+{
+    private readonly RichTextBox _richTextBox;
+
+    public LogPanelSink(RichTextBox richTextBox)
+    {
+        _richTextBox = richTextBox;
+    }
+
+    public void Emit(LogEvent logEvent)
+    {
+        var message = logEvent.RenderMessage();
+        var timestamp = logEvent.Timestamp.ToString("HH:mm:ss");
+        var level = logEvent.Level.ToString().ToUpperInvariant()[..4]; // INFO, WARN, ERRO, FATL
+        var text = $"{timestamp} [{level}] {message}";
+        var color = GetColor(logEvent.Level);
+
+        Application.Current?.Dispatcher.InvokeAsync(() =>
+        {
+            var para = new Paragraph(new Run(text) { Foreground = new SolidColorBrush(color) })
+            {
+                Margin = new Thickness(0)
+            };
+            _richTextBox.Document.Blocks.Add(para);
+            _richTextBox.ScrollToEnd();
+        });
+    }
+
+    private static Color GetColor(LogEventLevel level) => level switch
+    {
+        LogEventLevel.Warning => Colors.Orange,
+        LogEventLevel.Error or LogEventLevel.Fatal => Colors.Red,
+        _ => Colors.LimeGreen
+    };
+}