feat(annotations): add N/S/E/W edge midpoint handles to resize overlay

Extends the 4-corner L-bracket handles with 4 tick-mark edge handles
(short lines along each edge), enabling single-axis resize from any edge.
Updates applyHandleDrag to route each handle to the correct axis.

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

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

@@ -87,7 +87,7 @@ describe('AnnotationLayer', () => {
 			});
 
 			const handles = document.querySelectorAll('[data-handle]');
-			expect(handles).toHaveLength(4);
+			expect(handles).toHaveLength(8);
 		});
 
 		it('passes isResizable=false when annotation has a polygon', async () => {