fix(viewer): move delete button inside annotation bounds to prevent edge clipping

Repositioning from top:-8px/right:-8px to top:4px/right:4px ensures the
44px touch target stays fully within the annotation shape. Annotations drawn
near the top or right edge of the PDF page no longer risk the button being
obscured or inaccessible.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/AnnotationShape.svelte

@@ -130,8 +130,8 @@ let shapeStyle = $derived(
 			}}
 			style="
 				position: absolute;
-				top: -8px;
-				right: -8px;
+				top: 4px;
+				right: 4px;
 				min-width: 44px;
 				min-height: 44px;
 				display: flex;