fix(notes): let Shift+Enter break the line, and clear the leftovers around the editor

Shift+Enter did nothing because AcceptsReturn was only set from a style
setter and never reached the box. Setting it on the element is half the
fix: TextBox.OnKeyDown consumes Enter regardless of modifiers and runs
before any bubbling handler, so the split chain would have died in its
place. Enter is now taken in the tunnel phase and Shift+Enter is left to
the box.

Alongside, the notes list stops borrowing furniture it has no use for:
no "0 open tasks" subtitle, no show-completed and list-settings buttons,
and no details pane beside it — the editor spans that column instead of
leaving it empty.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ClaudeDo.Ui/Views/MainWindow.axaml

@@ -144,7 +144,7 @@
                     ResizeDirection="Columns"
                     ResizeBehavior="PreviousAndNext"/>
 
-      <Border Grid.Column="2" Classes="island" Margin="3">
+      <Border Grid.Column="2" Grid.ColumnSpan="{Binding TasksColumnSpan}" Classes="island" Margin="3">
         <islands:TasksIslandView DataContext="{Binding Tasks}"/>
       </Border>