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/Views/Islands/Detail/DescriptionStepsCard.axaml

@@ -103,7 +103,8 @@
           <!-- Description -->
           <Panel IsVisible="{Binding IsDescriptionSection}">
             <!-- Edit mode: raw TextBox -->
-            <TextBox Text="{Binding EditableDescription, Mode=TwoWay}"
+            <TextBox x:Name="DescriptionBox"
+                     Text="{Binding EditableDescription, Mode=TwoWay}"
                      AcceptsReturn="True"
                      TextWrapping="Wrap"
                      MinHeight="80"