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>

tests/ClaudeDo.Ui.Tests/Services/ClipboardImageTempStoreTests.cs

@@ -27,6 +27,20 @@ public class ClipboardImageTempStoreTests : IDisposable
         Assert.Equal(".png", Path.GetExtension(a));
     }
 
+    [Fact]
+    public void NewPngFileName_IsUniquePasteNameWithoutDirectory()
+    {
+        var now = new DateTime(2026, 9, 16, 10, 30, 0, 123, DateTimeKind.Utc);
+
+        var a = ClipboardImageTempStore.NewPngFileName(now);
+        var b = ClipboardImageTempStore.NewPngFileName(now);
+
+        Assert.NotEqual(a, b);
+        Assert.StartsWith("paste-20260916-103000123-", a);
+        Assert.EndsWith(".png", a);
+        Assert.Equal(a, Path.GetFileName(a));
+    }
+
     [Fact]
     public void DeleteOlderThan_RemovesOnlyFilesPastTheMaxAge()
     {