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/Core/Helpers/ExecuteQueryRetryHelper.cs

@@ -0,0 +1,45 @@
+using Microsoft.SharePoint.Client;
+using SharepointToolbox.Core.Models;
+
+namespace SharepointToolbox.Core.Helpers;
+
+public static class ExecuteQueryRetryHelper
+{
+    private const int MaxRetries = 5;
+
+    /// <summary>
+    /// Executes a SharePoint query with automatic retry on throttle (429/503).
+    /// Surfaces retry events via progress for user visibility ("Throttled — retrying in 30s…").
+    /// </summary>
+    public static async Task ExecuteQueryRetryAsync(
+        ClientContext ctx,
+        IProgress<OperationProgress>? progress = null,
+        CancellationToken ct = default)
+    {
+        int attempt = 0;
+        while (true)
+        {
+            ct.ThrowIfCancellationRequested();
+            try
+            {
+                await ctx.ExecuteQueryAsync();
+                return;
+            }
+            catch (Exception ex) when (IsThrottleException(ex) && attempt < MaxRetries)
+            {
+                attempt++;
+                int delaySeconds = (int)Math.Pow(2, attempt) * 5; // 10, 20, 40, 80, 160s
+                progress?.Report(OperationProgress.Indeterminate(
+                    $"Throttled by SharePoint — retrying in {delaySeconds}s (attempt {attempt}/{MaxRetries})…"));
+                await Task.Delay(TimeSpan.FromSeconds(delaySeconds), ct);
+            }
+        }
+    }
+
+    private static bool IsThrottleException(Exception ex)
+    {
+        var msg = ex.Message;
+        return msg.Contains("429") || msg.Contains("503") ||
+               msg.Contains("throttl", StringComparison.OrdinalIgnoreCase);
+    }
+}