refactor(annotations): replace 8-square handles with 4 corner L-brackets

- 4 corner-only handles (nw/ne/sw/se), no edge midpoints
- Each handle renders as two short perpendicular lines meeting at the corner
  (10px arms, navy, square linecap) — no fill, no box
- Thin dashed selection border added to SVG overlay to signal edit mode
- Simplify applyHandleDrag to remove dead n/s/e/w branches

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

frontend/src/lib/components/AnnotationShape.svelte.test.ts

@@ -44,7 +44,7 @@ describe('AnnotationShape', () => {
 			});
 
 			const handles = document.querySelectorAll('[data-handle]');
-			expect(handles).toHaveLength(8);
+			expect(handles).toHaveLength(4);
 		});
 	});
 });