feat(details): attach clipboard image on Ctrl+V in the task description

Extracts the clipboard bitmap read into a shared ClipboardBitmap helper
(also used by the terminal view); the description TextBox now routes an
image paste through AddFilesAsync as a unique paste-*.png attachment.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Services/ClipboardBitmap.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Threading.Tasks;
+using Avalonia.Controls;
+using Avalonia.Input;
+using Avalonia.Media.Imaging;
+
+namespace ClaudeDo.Ui.Services;
+
+/// <summary>Reads a bitmap off the clipboard; shared by the terminal view and the description editor.</summary>
+public static class ClipboardBitmap
+{
+    /// <summary>The clipboard bitmap (caller disposes), or null if there is none or the read failed.</summary>
+    public static async Task<Bitmap?> TryGetAsync(TopLevel? topLevel)
+    {
+        if (topLevel?.Clipboard is not { } clipboard) return null;
+        try
+        {
+            using var data = await clipboard.TryGetDataAsync();
+            return data is null ? null : await AsyncDataTransferExtensions.TryGetBitmapAsync(data);
+        }
+        catch (Exception)
+        {
+            return null;
+        }
+    }
+}

src/ClaudeDo.Ui/Services/ClipboardImageTempStore.cs

@@ -22,8 +22,11 @@ public static class ClipboardImageTempStore
 
     /// <summary>Test seam for <see cref="NewPngPath(DateTime)"/> — takes the target dir explicitly
     /// instead of the real app data root.</summary>
-    internal static string NewPngPath(string dir, DateTime utcNow) =>
-        Path.Combine(dir, $"paste-{utcNow:yyyyMMdd-HHmmssfff}-{Guid.NewGuid():N}.png");
+    internal static string NewPngPath(string dir, DateTime utcNow) => Path.Combine(dir, NewPngFileName(utcNow));
+
+    /// <summary>A fresh, collision-free <c>paste-….png</c> file name (also used as an attachment name).</summary>
+    public static string NewPngFileName(DateTime utcNow) =>
+        $"paste-{utcNow:yyyyMMdd-HHmmssfff}-{Guid.NewGuid():N}.png";
 
     /// <summary>Test seam for <see cref="DeleteOlderThan(TimeSpan,DateTime)"/> — takes the target dir
     /// explicitly instead of the real app data root.</summary>